Honeycomb filter

ABSTRACT

A honeycomb filter includes a plurality of cells, porous cell walls, and an oxidation catalyst. The plurality of cells include exhaust gas introduction cells and exhaust gas emission cells. The oxidation catalyst is supported inside the porous cell walls in an amount of 5 to 60 g/L. The exhaust gas emission cells have an average cross sectional area larger than an average cross sectional area of the exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction. A total volume of the exhaust gas introduction cells is larger than a total volume of the exhaust gas emission cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2013-159936, filed Jul. 31, 2013. The contents ofthis application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a honeycomb filter.

Background Art

Particulates (hereinafter also referred to as PMs or soot) such as sootin exhaust gas discharged from internal combustion engines includingdiesel engines cause damage to environment and human bodies, and thesedays people have paid attention to this problem. Since exhaust gascontains toxic gas components such as CO, HC, and NOx, people also worryabout the influences of the toxic gas components on the environment andhuman bodies.

To overcome this problem, various filters having honeycomb structures(honeycomb filters) formed of porous ceramics such as cordierite andsilicon carbide have been proposed as exhaust gas purifying apparatus.Such honeycomb filters are connected to internal combustion engines tocapture PMs in exhaust gas, or to convert the toxic gas components suchas CO, HC, and NOx in the exhaust gas into nontoxic gas.

In addition to CO in the exhaust gas, it is also necessary to purify COgenerated during combustion of PMs captured in the honeycomb filter. JP2008-284542 A discloses a honeycomb filter which supports an oxidationcatalyst on a predetermined area for efficiently converting CO.

For enhancing the fuel economy of internal combustion engines andavoiding troubles derived from an increase in the pressure loss duringoperation, various honeycomb filters have been proposed including thosein which the initial pressure loss is lowered by improvement of the cellstructure and those in which the rate of increase in the pressure lossis low when a certain amount of PM is accumulated.

JP 2003-284542 A, WO 2004/024294, U.S. Pat. No. 4,417,908, and WO2007/134897 disclose such filters.

JP 2008-284542 A discloses a honeycomb filter including: an outlet-sidecatalyst supporting area formed from an end face on a gas outlet sidetoward an end face on a gas inlet side; an inlet-side catalystsupporting area formed from an end face on the gas inlet side toward anend face on the gas outlet side; and a catalyst unsupporting area withno catalyst supporting layer, the catalyst unsupporting area beingbetween the outlet-side catalyst supporting area and the inlet-sidecatalyst supporting area. The catalyst unsupporting area has a higherthermal conductivity than the outlet-side catalyst supporting area andthe inlet-side catalyst supporting area. Ratios or the length of theoutlet-side catalyst supporting area, the length of the inlet-sidecatalyst supporting area, and the length of the catalyst unsupportingarea, relative to the entire length or the honeycomb filter in thelongitudinal direction satisfy a predetermined relationship.

FIG. 19A is a perspective view schematically illustrating a honeycombfilter disclosed in WO 2004/024294. FIG. 19B is a perspective viewschematically illustrating a honeycomb fired body forming the honeycombfilter.

As shown in FIGS. 19A and 19B, WO 2004/024294 discloses a honeycombfilter 90 that includes a plurality of honeycomb fired bodies 100combined with one another with adhesive layers 105 residingtherebetween, and an periphery coat layer 106 formed on the periphery ofthe combined honeycomb fired bodies, wherein the honeycomb fired bodies100 each include exhaust gas introduction cells 102 each having an openend at an exhaust gas introduction side and a plugged end at an exhaustgas emission side, and exhaust gas emission cells 101 each having anopen end at the exhaust gas emission side and a plugged end at theexhaust gas introduction side; the exhaust gas emission cells 101 eachhave a square cross section perpendicular to the longitudinal directionof the cells; the exhaust gas introduction cells 102 each have anoctagonal cross section perpendicular to the longitudinal direction ofthe cells; and the exhaust gas emission cells 101 and the exhaust gasintroduction cells 102 are alternately (in a grid-like pattern)arranged.

Hereinafter, in the explanation of the embodiments of the presentinvention and background arts, a cell having an open end at an exhaustgas emission side and a plugged end at an exhaust gas introduction sideis simply described as an exhaust gas emission cell. Moreover, a cellhaving an open end at an exhaust gas introduction side and a plugged endat an exhaust gas emission side is simply described as an exhaust gasintroduction cell, a first exhaust gas introduction cell, or a secondexhaust gas introduction cell.

The term just described as “cell” means both of an exhaust gas emissioncell and an exhaust gas introduction cell.

Moreover, cross sections perpendicular to the longitudinal direction ofcells including exhaust gas introduction cells, exhaust gas emissioncells, or the like are simply described as cross sections of the exhaustgas introduction cells, exhaust gas emission cells, or the like.

FIG. 20A is a perspective view schematically illustrating a honeycombfilter disclosed in U.S. Pat. No. 4,417,908. FIG. 20B is a viewschematically illustrating an end face of the honeycomb filter.

U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 in which allcells have the same square cross-sectional shape as shown in FIGS. 20Aand 20B. In the honeycomb filter 110, exhaust gas emission cells 111each having an open end at an exhaust gas emission side and a pluggedend at an exhaust gas introduction side are adjacently surrounded fullyby exhaust gas introduction cells 112 and 114 each having an open end atthe exhaust gas introduction side and a plugged end at the exhaust gasemission side across cell walls 113. In the cross section, a side of theexhaust gas introduction cell 112 faces the exhaust gas emission cellill across the cell wall 113, whereas the corners of the exhaust gasintroduction cells 114 respectively face the corners of the exhaust gasemission cells 111. Thus, none of the sides forming the cross sectionsof the exhaust, gas introduction cells 114 faces the exhaust gasemission cells 111.

WO 2007/134897 discloses a honeycomb filter including exhaust gasintroduction cells and exhaust gas emission cells, wherein, an exhaustgas emission cell having a hexagonal cross section is surrounded by sixexhaust gas introduction cells each having a hexagonal cross section,and the cross-sectional areas of all the exhaust gas introduction cellsare larger than the cross-sectional areas of all the exhaust gasemission cells.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a honeycomb filterincludes a plurality of cells, porous cell walls, and an oxidationcatalyst. Exhaust gas is to flow through the plurality of cells. Theplurality of cells include exhaust gas introduction cells and exhaustgas emission cells. The exhaust gas introduction cells each have an openend at an exhaust gas introduction side and a plugged end at an exhaustgas emission side. The exhaust gas emission cells each have an open endat the exhaust gas emission side and a plugged end at the exhaust gasintroduction side. The porous cell walls define rims of the plurality ofcells. The oxidation catalyst is supported inside the porous cell wallsin an amount of 5 to 60 g/L. The exhaust gas introduction cells and theexhaust gas emission cells each have a uniform cross sectional shapeexcept for a plugged portion in a cross section perpendicular to alongitudinal direction of the plurality of cells thoroughly from theexhaust gas introduction side to the exhaust gas emission side. Theexhaust gas emission cells have an average cross sectional area largerthan an average cross sectional area of the exhaust gas introductioncells in the cross section perpendicular to the longitudinal direction.A total volume of the exhaust gas introduction cells is larger than atotal volume of the exhaust gas emission cells.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

FIG. 1 is a perspective view schematically illustrating one example of ahoneycomb filter according to a first embodiment of the presentinvention.

FIG. 2A is a perspective view schematically illustrating one example ofa honeycomb fired body forming the honeycomb filter in FIG. 1. FIG. 2Bis an A-A line cross sectional view of the honeycomb fired body in FIG.2A.

FIG. 3A is an end face view illustrating an end of a honeycomb filteraccording to one embodiment of the present invention. FIG. 3B is anenlarged end face view illustrating an enlarged image of a part of anend face of a honeycomb filter according to one embodiment of thepresent invention.

FIGS. 4A and 4B are each a scanning electron microscope photograph (SEMphotograph) showing one example of the cross sections of cells.

FIGS. 5A and 5B are each a scanning electron microscope photograph (SEMphotograph) showing one example of the cross sectional shapes of cellsthat are different from the cells shown in FIGS. 4A and 4B.

FIGS. 6A to 6C are each an enlarged end face view illustrating anenlarged image of a part of an end face of a honeycomb filter accordingto one embodiment of the present invention.

FIG. 7 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 7 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections, repeated in the case where the second exhaust gasintroduction cells and the exhaust gas emission cells are octagonal andthe first exhaust gas introduction cells are square in the cross sectionof the cells, and also illustrates how the first exhaust gasintroduction cells and the second exhaust gas introduction cells areshared between the cell units (cell structure).

FIG. 8 is an explanatory diagram schematically illustrating a method formeasuring the initial pressure loss.

FIG. 9 is an explanatory diagram schematically illustrating a method formeasuring the pressure loss.

FIG. 10A is a graph showing a relation between PM capture amount and thepressure loss that are measured in Example 1 and Comparative Example 1.FIG. 10B is a graph showing a relation between the gas flow rate and theinitial pressure loss.

FIG. 11A is a perspective view schematically illustrating one example ofa honeycomb filter according to a second embodiment of the presentinvention. FIG. 11B is a B-B line cross sectional view of the honeycombfilter in FIG. 11A.

FIG. 12A is a perspective view schematically illustrating one example ofa honeycomb filter according to a third embodiment of the presentinvention. FIG. 12B is a perspective view illustrating a honeycomb firedbody forming the honeycomb filter.

FIG. 13A is a perspective view schematically illustrating one modifiedexample of the honeycomb fired body forming the honeycomb filteraccording to the third embodiment of the present invention. FIG. 13B isan end face view of the honeycomb fired body in FIG. 13A. FIG. 13C is anend face view illustrating another modified example of the honeycombfired body forming the honeycomb filter according to the thirdembodiment of the present invention.

FIG. 14A is an end face view schematically illustrating one example ofthe cell arrangement at an end face of a honeycomb fired body formingthe honeycomb filter according to a fourth embodiment of the presentinvention. FIG. 14B is an end face view illustrating one modifiedexample of the honeycomb fired body forming the honeycomb filteraccording to the fourth embodiment of the present invention. FIG. 14C isan end face view illustrating another modified example of the honeycombfired body forming the honeycomb filter according to the fourthembodiment of the present invention.

FIG. 15 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 15 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections, repeated in the case where the first exhaust gasintroduction cells, the second exhaust gas introduction cells, and theexhaust gas emission cells are square in the cross section of the cells,and also illustrates now the first exhaust gas introduction cells andthe second exhaust gas introduction cells are shared between the cellunits (cell structure).

FIG. 16 is an end face view schematically illustrating one example ofthe cell arrangement in an end face of a honeycomb fired body formingthe honeycomb filter according to a fifth embodiment of the presentinvention.

FIG. 17 is an end face view schematically illustrating one example of ahoneycomb fired body forming a honeycomb filter according to a sixthembodiment of the present invention.

FIG. 18A is an explanatory diagram schematically illustrating oneexample of a convex square cell shape. FIG. 18B is an explanatorydiagram schematically illustrating one example of a concave square cellshape. FIG. 18C is an explanatory diagram schematically illustrating oneexample of the concave square shape in which a vertex portion ischamfered. FIG. 18D is an explanatory diagram schematically illustratingone example of the convex square shape in which a vertex portion ischamfered.

FIG. 19A is a perspective view schematically illustrating a honeycombfilter disclosed in WO 2004/024294. FIG. 19B is a perspective viewschematically illustrating a honeycomb fired body forming the honeycombfilter.

FIG. 20A is a perspective view schematically illustrating a honeycombfilter disclosed in U.S. Pat. No. 4,417,908. FIG. 20B is an end faceview schematically illustrating an end face of the honeycomb filter.

FIG. 21A is a perspective view schematically illustrating a honeycombfilter according to comparative examples. FIG. 21B is a perspective viewschematically illustrating a honeycomb fired body forming the honeycombfilter shown in FIG. 21A.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention include a plurality of cellsfor allowing exhaust gas to flow therethrough, the cells includingexhaust gas introduction cells and exhaust gas emission cells, andporous cell walls defining rims of the plurality of cells, the exhaustgas introduction cells each having an open end at an exhaust gasintroduction side and a plugged end at an exhaust gas emission side, theexhaust gas emission cells each having an open end at the exhaust gasemission side and a plugged end at the exhaust gas introduction side,wherein an oxidation catalyst is supported inside the cell walls in anamount of 5 to 60 g/L; the exhaust gas introduction cells and theexhaust gas emission cells each have a uniform cross sectional shapeexcept for the plugged portion in a direction perpendicular to thelongitudinal direction of the cells thoroughly from the end at theexhaust gas introduction side to the end at the exhaust gas emissionside; the exhaust gas emission cells have a larger average crosssectional area in a direction perpendicular to the longitudinaldirection than the exhaust gas introduction cells in a directionperpendicular to the longitudinal direction; and the total volume of theexhaust gas introduction cells is larger than the total volume of theexhaust gas emission cells.

In the honeycomb filter of the embodiments of the present invention, theexhaust gas emission cells have a larger average cross sectional area ina direction perpendicular to the longitudinal direction than the exhaustgas introduction cells in a direction perpendicular to the longitudinaldirection. Thus, before accumulation of a certain amount of PMs in thehoneycomb filter, exhaust gas having flowed into the exhaust gasintroduction cells easily passes through the cell walls to flow into theexhaust emission cells. Moreover, the total volume of the exhaust gasintroduction cells is larger than the total volume of the exhaust gasemission cells, which increases the area of the inner walls of theexhaust gas introduction cells where exhaust gas can flow through. Thus,exhaust gas does not locally pass through the cell walls at a high rate,and easily contacts with the catalyst supported on the cell walls.Hence, the honeycomb filter highly efficiently purifies HC and CO inexhaust gas. Furthermore, even immediately after regeneration treatment,the honeycomb filter allows exhaust gas to easily pass inside the cellwalls in a direction parallel with the inner walls in the vicinity ofthe end at the exhaust gas introduction side where PMs are first burned.Exhaust gas easily contacts with the catalyst supported on the cellwalls. Hence, the honeycomb filter highly efficiently purifies COgenerated by combustion of PMs.

The expression “an oxidation catalyst is supported inside the cellwalls” herein means that an oxidation catalyst is supported on the cellwalls in a manner that, in an observation of a cross section of the cellwalls forming the honeycomb filter with a microscope (e.g., scanningelectron microscope (SEM)), the supported catalyst exists inside theoutlines of the cell walls. Although, the oxidation catalyst may besupported on the surface (outside or on the outlines of the cell walls)of the cell walls, the oxidation catalyst supported on the surface ofthe cell walls should not be considered the oxidation catalyst supportedinside the cell walls. For not supporting an excessive amount of anoxidation catalyst, the amount of the oxidation catalyst supported onthe surface of the cell walls is preferably as small as possible.Desirably, no oxidation catalyst is supported on the surface of the cellwalls.

The present inventors see that the pressure loss occurs due to (a)inflow resistance caused by exhaust gas flowing into the honeycombfilter, (b) flow-through resistance in the exhaust gas introductioncells, (c) passage resistance in the cell walls, (d) passage resistancecaused by exhaust gas upon passing through a layer of accumulated PMs,(e) flow-through resistance in the exhaust gas emission cells, and (f)outflow resistance caused by exhaust gas flowing out of the honeycombfilter. The inventors found out that, before accumulation of a certainamount of PMs in the honeycomb filter, exhaust gas having flowed intoexhaust gas introduction cells is likely to flow long inside the exhaustgas introduction cells toward the end of the honeycomb filter and passthrough a local part of the cell walls relatively near the end of thehoneycomb filter to flow into exhaust gas emission cells. In each cellof the honeycomb filter of the embodiments of the present invention, (b)passage resistance in the exhaust gas introduction cells is greater than(e) passage resistance in the exhaust gas emission cells, and also (f)outflow resistance caused by exhaust gas flowing out of the honeycombfilter is reduced. Thus, exhaust gas having flowed into the exhaust gasintroduction, cells is easily allowed to pass through the entire cellwalls in the longitudinal direction to flow into the exhaust gasemission cells.

Immediately after regeneration treatment, (d) passage resistance causedby exhaust gas upon passing through a layer of accumulated PMs issmaller in the vicinity of the end at the exhaust gas introduction sidewhere PMs are first burned, and thus exhaust gas preferentially passesfrom the exhaust gas introduction cells into the exhaust gas emissioncells at the vicinity. The exhaust gas introduction cells, which have alarger total volume, have a large filtration area. Thus, afteraccumulation of PMs, exhaust gas is allowed to pass inside the cellwalls also in a direction parallel with the inner walls of the exhaustgas introduction cells, and can efficiently contacts with the catalystsupported on the cell walls. Hence, the honeycomb filter can have ahigher ability of purifying CO caused by combustion of PMs as well as HCand CO in exhaust gas.

The honeycomb filter of the embodiments of the present inventionsupports an oxidation catalyst in an amount of 5 to 60 g per liter of anapparent volume of the honeycomb filter.

The expression “supports an oxidation catalyst” herein means that anoxide having a high specific surface area of more than 50 m²/g, such asalumina, silica, ceria, or zirconia, is supported as a catalyst support,and further a noble metal, snob as platinum (Pt), Rhodium (Rh), orPalladium (Pd), is supported on the catalyst support in the honeycombfilter. The amount of 5 to 60 g/L means the total amount of the oxideand the noble metal supported. The amount of the noble metal supportedis preferably 0.5 to 5 g per liter of an apparent volume of thehoneycomb filter.

If an oxidation catalyst is supported inside the cell walls in an amountof 5 to 60 g/L, a sufficient CO and HC purification ratio can beachieved. Also an increase in the pressure loss of the honeycomb filterdue to supporting of the catalyst can be minimized.

An amount of an oxidation catalyst supported of less than 5 g/L is toosmall, which makes it difficult to sufficiently purify HC and CO inexhaust gas and CO generated by combustion of PMs. An amount of anoxidation catalyst supported of more than 60 g/L is too large. Thus,exhaust gas hardly passes through pores so that the pressure lossincreases.

Preferably, in the honeycomb filter of the embodiments of the presentinvention, in addition to the above structure, each exhaust gas emissioncell is adjacently surrounded fully by the exhaust gas introductioncells across the porous cell walls;

the exhaust gas introduction cells include first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga larger cross sectional area than each first exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells;

each exhaust gas emission cell has an equal or larger cross sectionalarea than, each second exhaust gas introduction cell in a directionperpendicular to the longitudinal direction of the cells;

in the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas introduction cells and the exhaust gas emissioncells are each polygonal; and

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, a sideforming the cross sectional shape of each second exhaust gasintroduction cell faces one of the exhaust gas emission cells, and theside of the first exhaust gas introduction cell is longer than the sideof the second exhaust gas introduction cell, or

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, and noneof the sides forming the cross sectional shape of each second exhaustgas introduction cell faces the exhaust gas emission cells.

The honeycomb filter having the above configuration can have a smallerinitial pressure loss compared to conventional honeycomb filters. Evenafter accumulation of a large amount of PMs on the cell walls, the rateof increase in the pressure loss is small. As a result, the pressureloss is improved over the entire period from the initial stage to afteraccumulation of PMs in close to the limit amount.

With regard to the resistances (a) to (f) which are factors of pressureloss, the study of the inventors has revealed that the factors (c), (e),and (f) are controlling factors of the initial, pressure loss thatoccurs before accumulation of PMs, and that the factors (a), (b), and(d) are controlling factors of the transitional pressure loss thatoccurs after accumulation of a certain amount of PMs. One of thecontrolling factors of the initial pressure loss is not the factor (b)flow-through resistance in the exhaust gas introduction cells but thefactor (e) flow-through resistance in the exhaust gas emission cellsbecause the aperture ratio of the honeycomb filter based on the exhaustgas emission cells is smaller than the aperture ratio of the honeycombfilter based on the exhaust gas introduction cells. Similarly, theinventors consider the factor (f) outflow resistance caused by exhaustgas flowing out of the honeycomb filter, not the factor (a) inflowresistance caused by exhaust gas flowing into the honeycomb filter, asone of the controlling factors of the initial pressure loss because theysuppose that the resistance due to compression of the gas is smallerthan the resistance due to disturbance of emission of the exhaust gascaused by eddying flow of the gas that occurs near the end of the cellswhen the gas rapidly expands upon emission from the cells.

Since the honeycomb filter of the embodiments of the present inventionhas the exhaust gas introduction cells arranged to fully surround theexhaust gas emission cells across porous cell walls, there are no otheropenings from which exhaust gas can flow out around each exhaust gasemission cell on the exhaust gas emission side. This structure is lesslikely to cause large eddying flow or tire like upon emission of exhaustgas. This lowers the outflow resistance of the factor (f). Moreover,since the entire area of the cell walls can be used for filtration, PMsare likely to be thinly and uniformly accumulated on the inner walls ofthe exhaust gas introduction cells, lowering the (d) passage resistancethrough a layer of accumulated PMs. Thus, in the provided honeycombfilter, the pressure loss is small at the initial stage and is lesslikely to increase even after accumulation of PMs.

The phrase “cross sectional shape of a cell” herein refers to a shapeformed by an inner cell wall of the exhaust gas emission cell, firstexhaust gas introduction cell, or second exhaust gas introduction cellin the direction perpendicular to the longitudinal direction of thecell.

The phrase “cross sectional area of a cell” herein refers to an area ofa cross sectional shape formed by an inner cell wall of the exhaust gasemission cell, first exhaust, gas introduction cell, or second exhaustgas introduction cell in a cross section perpendicular to thelongitudinal direction of the cell. The term “inner cell wall” refers toa surface on the inner side of a cell among surfaces of cell walls.

Moreover, the term “side” herein refers to a segment between vertices ofa polygon in the case where cross sectional shapes formed by inner cellwalls of the exhaust gas emission cells, the first exhaust gasintroduction cells, or the second exhaust gas introduction cells arepolygons in a direction perpendicular to the longitudinal direction ofthe cells.

The phrase “volume of a cell” refers to a volume of a part surrounded byinner walls except for the plugged portion of the cell. The term “totalvolume” refers to the total sum of the volumes. The volume of the cellis calculated by multiplying the cross sectional area of the cell andthe length of the cell excluding the length of the plugged portion.

Furthermore, the term “length of a side” means the length of thesegment. In the case of roundly-cornered shapes with the vertex portionsformed by curved lines (so-called chamfered shape), the length of a sidemeans the length of a straight line excluding the curved line portionsfor the following reasons.

In the case where the vertex portions are formed by curved lines, thecell walls separating the cells are thick in the curve portions, andthus the curve portions have high passage resistance. This causesexhaust gas to preferentially flow into straight line portions, and thusthe length of the straight portions needs to be controlled. Hence, it isreasonable to exclude the curve portions from consideration.

Provided that the straight portions of a polygon are hypotheticallyextended, and intersections of the hypothetical straight lines are givenas hypothetical vertices, the length of the straight portion of the sideexcluding the curve portion is preferably not less than 80% the lengthof a hypothetical side given by connecting the hypothetical vertices. Inthe case of the cell having a polygonal cross sectional shape in whichthe sides have not less than 80% the length of the hypothetical sides, amain-channel-switching effect, which is a functional effect of theembodiments of the present invention, can be achieved by adjusting thelength of the sides.

In the honeycomb filter according to the embodiments of the presentinvention, a side forming the cross sectional shape of a first exhaustgas introduction cell or a second exhaust gas introduction cell isconsidered to face an exhaust gas emission cell when the followingcondition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of afirst exhaust gas introduction cell or a second exhaust gas introductioncell is given from the side to outside the first exhaust gasintroduction cell or the second exhaust gas introduction cell, theperpendicular bisector crosses a shape region defined by the inner cellwall of an exhaust gas emission cell which is adjacent to the firstexhaust gas introduction cell or a second exhaust gas introduction cellacross a cell wall.

In such a case, a first exhaust gas introduction cell or second exhaustgas introduction cell having a side facing an exhaust gas emission cellis considered to face the exhaust gas emission cell.

In the honeycomb filter according to the embodiments of the presentinvention, a side forming the cross sectional shape of an exhaust gasemission cell is considered to face a first exhaust gas introductioncell or a second exhaust gas introduction cell when the followingcondition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of anexhaust gas emission cell is given from the side to outside the exhaustgas emission cell, the perpendicular bisector crosses a shape regiondefined by the inner cell wall of a first exhaust gas introduction cellor a second exhaust gas introduction cell which is adjacent to theexhaust gas emission cell across a cell wall.

In such a case, an exhaust gas emission cell having a side facing afirst exhaust gas introduction cell or a second exhaust gas introductioncell is considered to face the first exhaust gas introduction cell orthe second exhaust gas introduction cell.

Moreover, in the honeycomb filter according to the embodiments of thepresent invention, a side forming a first exhaust gas introduction cellis considered to face a second exhaust gas introduction cell when thefollowing condition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal, direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of afirst exhaust gas introduction cell is given from the side to outsidethe first exhaust gas introduction cell, the perpendicular bisectorcrosses a shape region defined by the inner cell, well of a secondexhaust gas introduction cell which is adjacent to the first exhaust gasintroduction cell across a cell wall.

In such a case, a first exhaust gas introduction cell having a sidefacing a second exhaust gas introduction cell, is considered to face thesecond exhaust gas introduction cell.

Furthermore, in the honeycomb filter according to the embodiments of thepresent invention, a side forming a second exhaust gas introduction cellis considered to face a first exhaust gas introduction cell when thefollowing condition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon teamed by the inner cell wall of asecond exhaust gas introduction cell is given from the side to outsidethe second exhaust gas introduction cell, the perpendicular bisectorcrosses a shape region defined by the inner cell wall of a first exhaustgas introduction cell which is adjacent to the second exhaust gasintroduction cell across a cell wall.

In such a case, the second exhaust gas introduction cell having a sidefacing a first exhaust gas introduction cell is considered to face thefirst exhaust gas introduction cell.

In the honeycomb filter of the embodiments of the present invention, thethickness of a cell wall separating two cells is defined as follows.

Namely, in the cross section perpendicular to the longitudinal directionof cells, provided that a hypothetical straight line is given whichconnects geometric centers of gravity of cross sectional figures definedby the inner cell walls of two cells, the length of a segment of thestraight line overlapping the cell wall area is defined as the thicknessof the cell wall. Although cells are void, the centers of gravity hereinrefer to geometric centers of gravity of cross sectional figures definedby inner cell walls. Thus, the center of gravity can be defined even forcross sectional figures of void such as cells.

The word “adjacent” herein is equivalent to the word “adjacent” inJapanese. The word “adjacent” is used not only for a case where exhaustgas introduction cells are arranged to face exhaust gas emission cellsacross porous cell walls, but also for a case where exhaust gasintroduction cells are not facing but arranged diagonally to exhaust gasemission cells across porous cell walls. In Japanese, the expression“Lattices are diagonally adjacent to each other” is accepted as anexemplary expression using “adjacent”.

FIG. 1 is a perspective view schematically illustrating one example of ahoneycomb filter according to a first embodiment of the presentinvention. FIG. 2A is a perspective view schematically illustrating oneexample of a honeycomb fired body forming the honeycomb filter inFIG. 1. FIG. 2B is an A-A line cross sectional view of the honeycombfired body in FIG. 2A. FIG. 3A is an end face view illustrating an endof a honeycomb filter according to one embodiment of the presentinvention. FIG. 3B is an enlarged end face view illustrating an enlargedimage of a part of an end face of a honeycomb filter according to oneembodiment of the present invention.

The case where exhaust gas introduction cells and exhaust gas emissioncells each have a polygonal cross sectional shape and are arranged toface each other across porous cell walls is specifically illustrated inFIGS. 2A to 3B. In FIGS. 2A to 3B, second exhaust gas introduction cells14 face exhaust gas emission cells 11 across porous cell walls 13. In acase where exhaust gas introduction cells and exhaust gas emission cellseach have a round or elliptical cross sectional shape and a singleporous cell wall is formed by curves of the cross sectional shapes of anexhaust gas introduction cell and an exhaust gas emission cell (a casewhere a curve of the inner wall of an exhaust gas emission cell and acurve of the inner wall of an exhaust gas introduction cell form thefront side and the rear side of a single cell wall in athree-dimensional view), the exhaust gas introduction cell and theexhaust gas emission cell are considered to be arranged to face eachother across a porous cell wall according to the case where exhaust gasintroduction cells and exhaust gas emission cells each have a polygonalcross sectional shape. Specifically, such a case is illustrated in FIG.16. In FIG. 16, second exhaust gas introduction cells 54 are consideredto face exhaust gas emission cells 51 across porous cell walls 53.

The case where exhaust gas introduction cells and exhaust gas emissioncells each have a polygonal cross sectional shape and are not arrangedto face each other but arranged diagonally across porous cell walls isspecifically illustrated in FIGS. 14A to 14C. In FIGS. 14A to 14C,second exhaust gas introduction cells 44 and exhaust gas emission cells41 do not face each other and are arranged diagonally across porous cellwalls 43.

In a case where exhaust gas introduction cells and exhaust gas emissioncells each have a shape formed by curved lines, except for round orelliptical shape, an intersection of two curved lines is considered tobe a vortex and a curved line between two vertices is considered to be aside. Here, a side (curved line) of an exhaust gas emission cell orexhaust gas introduction cell is considered to face an exhaust gasintroduction cell or exhaust gas emission cell when the followingcondition is satisfied. Provided that a hypothetical perpendicular linewhich bisects a segment between vertices at the both ends of a side(curved line) forming the cross sectional shape of an exhaust gasemission cell or an exhaust gas introduction cell (A) is given from theside toward the outside the exhaust gas emission cell or the exhaust gasintroduction cell (A), the perpendicular bisector crosses a shape regiondefined by the inner cell wall of an exhaust gas introduction cell or anexhaust gas emission cell (B) that is closest to the cell (A) across acell wall. Moreover, the exhaust gas emission cell or the exhaust gasintroduction cell (A) is described to face the exhaust gas introductioncell or the exhaust gas emission cell (B). In the case of the vertexportions formed by curved lines (so-called chamfered shape), the curvedlines are extended, and the hypothetical intersection of the extendedlines is considered to be a vertex.

In a case where exhaust gas introduction cells and exhaust gas emissioncells each have a shape formed by curved lines, except for round orelliptical shape, a case where exhaust gas introduction cells endexhaust gas emission cells do not face each other and are arrangeddiagonally across porous cell walls is specifically illustrated in FIG.17. In FIG. 17, exhaust gas introduction cells 64 and exhaust gasemission cells 61 do not face each other and are arranged diagonallyacross porous cell walls 63.

The sentence “each of the exhaust gas emission cells is adjacentlysurrounded fully by the exhaust gas introduction cells across the porouscell walls” can encompass “each exhaust gas emission cell is enclosedall around by the adjacent exhaust gas introduction cells across theporous cell walls” in the present application.

Here, the phrase “arranges diagonally” refers to the arrangement inwhich exhaust gas introduction cells and exhaust gas emission cells donot face each other and satisfy the following condition. In a crosssection perpendicular to the longitudinal direction of an exhaust gasemission cell, a hypothetical segment between the geometrical center ofgravity and a vertex (in the case of chamfered vertex portions, sides(straight or curved lines) forming the cross sectional figure arehypothetically extended and an intersection of the extended lines isconsidered to be a vertex) of a cross sectional figure formed by theinner cell wall of the exhaust gas emission cell is provided. In a crosssection perpendicular to the longitudinal direction of an exhaust gasintroduction cell, a hypothetical segment between the geometrical centerof gravity and a vertex (in the case of chamfered vertex portions, sides(straight or curved lines) forming the cross sectional figure arehypothetically extended and an intersection of the extended lines isconsidered to be a vertex) of a cross sectional figure formed by theinner cell wall of the exhaust gas introduction cell is provided. Here,the provided hypothetical segments are parallel with each other oroverlapped with each other. It is to be noted that, if a pair ofhypothetical lines among plural hypothetical lines is parallel oroverlapped with each other, the other hypothetical lines may be acrosswith each other at a predetermined angle (e.g., 90°).

In the description of the term “adjacent”, the term “exhaust gasintroduction cell” refers to both the first exhaust gas introductioncell and the second exhaust gas introduction cell.

Next, a side which faces a cell and the thickness of a cell wallseparating two cells are explained below based on figures.

FIG. 3B is an enlarged end face view illustrating an enlarged image of apart of an end face of the honeycomb filter according to one embodimentof the present invention. FIG. 3B illustrates exhaust gas emission cells11, and first exhaust gas introduction cells 12 and second exhaust gasintroduction cells 14 surrounding the exhaust gas emission cells 11.

A side forming the cross sectional shape of a first exhaust gasintroduction cell 12 or a second exhaust gas introduction cell 14 isconsidered to face an exhaust gas emission cell 11 when the followingcondition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of the cells shown in FIGS.3A and 3B, a hypothetical perpendicular line (hereinafter referred to asa perpendicular bisector) which bisects a side 12 a of a polygon formedby the inner cell wall of a first exhaust gas introduction cell 12 or aside 14 a of a polygon formed by the inner cell wall of a second exhaustgas introduction cell 14 is given from the side to outside the firstexhaust gas introduction cell 12 or the second exhaust gas introductioncell 14, the perpendicular bisector A or the perpendicular bisector Bcrosses a shape region (side 11 a, side 11 b) defined by the inner cellwall of an exhaust gas emission cell 11 which is adjacent to the side 12a of the first exhaust gas introduction cell 12 or the side 14 a of thesecond exhaust gas introduction cell 14 across a cell wall as shown inFIGS. 3A and 3B.

The reason why the crossing of the bisector is set as a condition forthe facing in the embodiments of the present invention is that thepassage resistance caused upon allowing exhaust gas to pass through ator around the center of the side in the length direction, i.e. at oraround the center of the cell wall separating the exhaust gasintroduction cell and the exhaust gas emission cell, represents apressure loss caused upon allowing exhaust gas to pass through theentire wall.

According to the embodiments of the present invention, in the case wherethe cross sectional shape defined by the inner cell wall of each exhaustgas emission cell, first exhaust gas introduction cell, or secondexhaust gas introduction cell is a polygon, and the vertex portions ofthe polygon are chamfered, i.e. formed by curved lines in the crosssection perpendicular to the longitudinal direction of the cells, thebisectors of respective sides are bisectors of segments excluding thecurved lines.

Moreover, in the case where the vertex portions are roundly-cornered,i.e. formed by curved lines, the curved lines are not included in thesides. In the case where the vertex portions are chamfered in the crosssectional shape, the sides forming the cross sectional shape arehypothetically extended, and intersections of the extended sides areconsidered as hypothetical vertices. Hence, the cross sectional shape istreated as a polygon.

This is based on the following intention. For manufacturing a honeycombfilter including cells having polygonal cross sectional shapes in thecross section perpendicular to the longitudinal direction of the cellsby extrusion molding, the vertex portions of the polygonal crosssectional shapes may be formed by curved lines to prevent concentrationof stress at the vertex portions. Such cross sections in which thevertex portions are formed by curved lines are considered as polygons.

The thickness of a cell wall separating two cells is defined as follows.

Namely, in the cross section perpendicular to the longitudinal directionof the cells shown in FIGS. 3A and 3B, provided that a hypothetical lineZ₁₄ is given which connects geometric centers of gravity of the crosssectional figures defined by the inner cell walls of the two cells (thecenter of gravity of the exhaust gas emission cell 11 is O₁₁, and thecenter of gravity of the second exhaust gas introduction cells is O₁₄ inFIGS. 3A and 3B), the length D of the segment of the line Z₁₄overlapping the cell wall area is defined as the thickness of the cellwall. Although cells are voids, the centers of gravity herein refer togeometric centers of gravity of cross sectional figures defined by innercell walls. Thus, the center of gravity can be defined even for crosssectional figures of voids such as cells.

The thickness of the cell wall is defined as above for the followingreasons. The passage resistance caused upon allowing gas to pass throughthe cell walls is the highest at a portion of the cell wall where thegas passes through at the highest flow rate, and the passage resistanceat the portion may represent the passage resistance of the cell wall.The flow rate of gas in the longitudinal direction of the honeycombfilter is the highest at positions corresponding to the geometriccenters of gravity of the cross sectional shapes defined by the innercell walls. The flow rate concentrically decreases from the center ofgravity toward the circumference of the cross sectional shape of thecells. Thus, the flow rate of gas passing through a cell wall is thehighest at as intersection of the cell wall and a line connecting thecenter of gravity of an exhaust gas introduction cell and the center ofgravity of an exhaust gas emission cell. In view of the pressure loss asdescribed above, it is reasonable to define a length D of the segment ofa portion where a straight line connecting the centers of gravityoverlaps the cell wall area as the thickness of a cell wall.

According to the embodiments of the present invention, electronmicroscope pictures are used for measurement of the length of sides andthe thickness of cell walls, and identification of the cross sectionalshapes of cells. The electron microscope pictures are taken with anelectron microscope (FE-SEM: High resolution field emission scanningelectron microscope S-4800, manufactured by Hitachi High-TechnologiesCorporation).

The electron microscope pictures are preferably those taken by theelectron microscope at a magnification of 30×. This is because, at amagnification of 30×, irregularities due to particles or pores on thesurface (inner wall) of cell walls defining the rims of cells do notdisturb identification of the cross sectional shapes of the cells,measurement of the lengths of sides, thicknesses of cell walls, andcross sectional area of the cells. Also, at a magnification of 30×, thecross sectional shapes of the cells can be identified, and the lengthsof sides, thicknesses of cell walls, and cross sectional area of thecells can be measured.

In other words, the lengths of the respective sides of the cell aremeasured using the scale of the electron microscope photographs based onthe above definitions of the length of the cells and the thickness ofthe cell walls. The cross sectional area is arithmetically determinedbased on the obtained values including the length of the cells. Ifarithmetic calculation of the cross sectional area is complicated, thecross sectional area can also be determined by cutting a square piece (asquare with a side having a scale length) corresponding to a unit areaout of the electron microscope photograph, and weighing the cut-outpiece, while separately cutting the cross section of the cell out alongthe cross sectional shape of the cell (along curved lines in the case ofa polygonal cross section with the vertex portions formed by curvedlines), and weighing the cut-out piece, and then calculating the crosssectional area of the cell based on the weight ratio.

For example, in FIG. 4A, the cross sectional shapes defined by the innerwalls of the exhaust gas emission cells and the second exhaust gasintroduction cells are octagons having the same cross sectional areafrom one another, and the cross sectional shapes defined by therespective inner walls of the first exhaust gas introduction cells aresquare (although the vertex portions have so-called roundly-corneredshape (i.e. formed by curved lines), the cross sectional shapes are eachconsidered as a square having four sides and four vertices at fourintersections of straight lines extended from the four sides forming thecross sectional shape in the embodiments of the present invention). Inthe photograph, a 500 μm scale is displayed. A square (corresponding toa unit area) having each side in a length corresponding to 500 μm in thephotograph is cut out of the photograph, and the cut-out piece isweighed. Next, the octagon and the square are cut out of the photograph(the four vertex portions formed by curved lines in the square are cutalong the curved lines), and the cut-out pieces are weighed. The crosssectional area is calculated based on the weight ratio between thecut-out piece and the 500 μm scale square. In the case of measuring onlythe cross sectional area ratio of the cells, the area ratio can beobtained directly from the weight ratio between the octagon and thesquare.

According to the embodiments of the present invention, the measurementof the lengths of the cells, the thicknesses of the cell walls, and thecross sectional areas can be changed from the above manual measurementto an electronic measurement by scanning the electron microscopephotograph as image data, or using the image data directly output fromthe electron microscope and entering the scale of the photograph. Themanual measurement and the electronic measurement are both based on thescale of the electron microscope image, and are in accordance with thesame principle. Thus, surely no discrepancies are found between themeasurement results of the respective measurements.

For example, the electronic measurement may be performed by using animage analysis and grain size distribution measurement software(Mac-View (Version 3.5), produced by Mountech Co. Ltd.). This softwaremeasures a cross sectional area by scanning an electron microscopephotograph as image data, or using the image data directly output fromthe electron microscope, entering the scale of the photograph, andspecifying the area along the inner wall of the cell. Moreover, thedistance between any two points in the image can be measured based onthe scale of the electron microscope photograph.

A photograph of the cross section of the cells is taken with theelectron microscope by cutting a filter in a direction perpendicular tothe longitudinal direction of the cells in prepare a 1 cm×1 cm×1 cmsample including the cut face and ultrasonic cleaning the cut section ofthe sample, or coating a filter with resin and cutting the coated filterin a direction perpendicular to the longitudinal direction of the cells,and then taking an electron microscope photograph of the sample. Theresin coating does not affect measurement of the lengths of she sides ofthe cells and the thicknesses of the cell walls.

FIGS. 4A and 4B are each a photograph showing one example of the shapeof the cross section of cells taken with a measuring microscope.

FIG. 4A reveals that the cross sectional shapes of the exhaust gasemission cells 11 and the second exhaust gas introduction cells 14 areeach octagonal. The cross sectional shape of the first exhaust gasintroduction cells 12 is square. The vertex portions of each firstexhaust gas introduction cell are formed by slightly curved lines;however, extension of the four sides, which are straight lines, of thefirst exhaust gas introduction cell 12 intersect at four intersectionsto form a square having the intersections as vertices. Thus, the crosssection of the cell is considered square according to the definition ofthe embodiments of the present invention.

Moreover, calculation with MAC-View (Version 3.5) reveals that the areaof the cross sectional shape (cross sectional area) of the exhaust gasemission cell 11 and the second exhaust gas introduction cell 14 is 2.14mm², and the area of the cross sectional shape (cross sectional area) ofthe first exhaust gas introduction cell 12 is 0.92 mm².

Furthermore, as shown in FIG. 4B, since the four vertex portions of thefirst exhaust gas introduction cell 12 are formed by curved lines, thelength of a side Ls facing the exhaust gas emission cell 11 among thesides forming the cross sectional shape of the first exhaust gasintroduction cell 12 is the length excluding the curved portions.Additionally, the length Lo of a side facing the exhaust gas emissioncell among the sides forming the cross sectional shape of the secondexhaust gas introduction cell 14 corresponds to the distance between thevertices of the octagon.

As described above, the lengths of the sides Ls and Lo, and the crosssectional area can be measured using the electron microscope photograph.

FIGS. 5A and 5B are each a scanning electron microscope photograph (SEMphotograph) showing one example of the cross sectional shapes of cellsthat are different from the cells shown in FIGS. 4A and 4B.

FIG. 5A shows that the cross sectional areas of the respective exhaustgas emission cell 41, the second exhaust gas introduction cell 44, andthe first exhaust gas introduction cells 42 are each in a shape in whichstraight lines hypothetically extended from the four sides having equallength perpendicularly intersect one another at intersections(vertices), and the vertex portions are formed by curved lines. Althoughthe vertex portions of the cross sectional shape of the cell are formedby curved lines, lines extended from the four straight lines formingeach cell intersect at four intersections. Supposing that theintersections are hypothetical vertices, the four distances between thevertices are the same from one another to form a square. Thus, the crosssectional shapes of the respective cells are considered square accordingto the definition of the embodiments of the present invention.

Moreover, as is understood from FIG. 5B, a perpendicular bisector of aside forming the first exhaust gas introduction cell 42 crosses theexhaust gas emission cell 41. Thus, the side forming the first exhaustgas introduction cell 42 faces the exhaust gas emission cell 41. Incontrast, a perpendicular bisector of a side forming the second exhaustgas introduction cell 44 does not intersect with the exhaust gasemission cell 41. Thus, the side forming the second exhaust gasintroduction cell 44 does not face the exhaust gas emission cell 41. Asdescribed above, whether a side forming the second exhaust gasintroduction cell 44 or the first exhaust gas introduction cell 42 facesthe exhaust gas emission cell 41 can be determined from the electronmicroscope photograph.

A convex square according to the embodiments of the present invention isa shape formed by four outwardly curved lines of the same length. Thesquare looks as if its sides bulge from the geometric center of gravityto the outside. A concave square is a shape formed by four inwardlycurved lines of the same length. The square looks as if its sidesconcave toward the geometric center of gravity.

In the cross section perpendicular to the longitudinal direction of thecells forming the honeycomb filter according to the embodiments of thepresent invention, the first exhaust gas introduction cells, the secondexhaust gas introduction cells, and the exhaust gas emission cells eachhave a uniform cross sectional shape thoroughly from the exhaust gasintroduction end to the exhaust gas emission end except for the pluggedportion. Namely, taking the first exhaust gas introduction cell as anexample, in a cross sectional view perpendicular to the longitudinaldirection, the cross sectional shape defined by the inner wall thereofhas the same shape at any part from the exhaust gas introduction end tothe exhaust gas emission end except for the plugged portion. The sameshape means a congruent shape, and excludes similar shapes. Namely, asimilar shape means a different shape. The explanation for the firstexhaust gas introduction cell also applies to the second exhaust gasintroduction cell and the exhaust gas emission cell. The pluggedportions are excluded because the cross sectional shape defined by theinner cell wall does not physically exist at the plugged portions due tothe presence of the plugs.

The honeycomb filter of the embodiments of the present invention cancomprehensively reduce the pressure loss thoroughly from the initialstage to the stage after accumulation of PMs in close to the limitamount, as compared to conventional honeycomb filters.

In consideration of the above-described pressure loss broken down torespective resistance components, the flow-through resistance and theoutflow resistance need to be reduced for reducing the initial pressureloss. Thus, the cross sectional area of the exhaust gas emission cellsneeds to be equal to or relatively larger than the cross sectional areaof the exhaust gas introduction cells for suppressing the rapidexpansion. Wide and thin accumulation of PMs is necessary for reducingthe transitional pressure loss. Thus, the cross sectional area of theexhaust gas introduction cells needs to be relatively larger than thecross sectional area of the exhaust gas emission cells.

It has been considered impossible to reduce both of the transitionalpressure loss and the initial pressure loss. The inventors of thepresent invention further studied and completed the present inventiondescribed below.

That is, exhaust gas is preferentially introduced firstly into the firstexhaust gas introduction cells when the honeycomb filter has thefollowing structures: two kinds of the exhaust gas introduction cellsincluding the exhaust gas introduction cells each having a larger crosssectional area (second exhaust gas introduction cells) and the exhaustgas introduction cells each having a smaller cross sectional area (firstexhaust gas introduction cells) are employed as the exhaust gasintroduction cells; each exhaust gas emission cell has an equal orlarger cross sectional area than each second exhaust gas introductioncell; each exhaust gas emission cell is fully surrounded by the twokinds of the exhaust gas introduction cells; and the length of the innerwall separating the first exhaust gas introduction cell and the exhaustgas emission cell is relatively longer than the length of the inner wallseparating the second exhaust gas introduction cell and the exhaust gasemission cell, or the thickness of the wall separating the first exhaustgas introduction cell and the exhaust gas emission cell is relativelysmaller than the thickness of the wall separating the second exhaust gasintroduction cell and the exhaust gas emission cell.

The wall separating the first exhaust gas introduction cell and theexhaust gas emission cell has a larger passage area (in the case ofpolygonal cells, sides are long in the cross sectional shape) or thethickness of the wall is smaller. Exhaust gas can thus pass through eachan advantageous wall so that the passage resistance of the factor (c)can be reduced. Also, the flow-through resistance of the factor (e) canbe reduced as the cross sectional area of the exhaust gas emission cellsis relatively larger than the cross sectional area of the first exhaustgas introduction cells. Namely, both of the passage resistance of thefactor (c) and the flow-through resistance of the factor (e) can bereduced, and thereby the initial pressure loss can be reduced. Afteraccumulation of a certain amount of PMs, since the cross sectional areaof the first exhaust gas introduction cells is smaller than the crosssectional area of the second exhaust gas introduction cells, an increasein the passage resistance occurs earlier in a layer of the accumulatedPMs in the first exhaust gas introduction cells. This leads to“switching” of the main flow channel of exhaust gas in a manner that alarger amount of the exhaust gas is naturally (i.e. autonomously)introduced to the second exhaust gas introduction cells. Consequently,PMs are widely and thinly accumulated in the second exhaust gasintroduction cells each having a large cross sectional area. Hence, bothof the flow-through, resistance of the factor (b) and the passageresistance of the factor (d) can be reduced so that the transitionalpressure loss can be reduced even after accumulation of PMs.

As described above, the embodiments of the present invention haveachieved a surprising effect, which has been considered impossible, ofreducing both of the transitional pressure loss and the initial pressureloss by the autonomous snitching of the main flow channel.

The aforementioned effect of reducing both of the initial pressure lossand the transitional pressure loss by “switching” of the main flowchannel to which a larger amount of exhaust gas is introduced is exertedonly when all the aforementioned features work integrally. Suchstructures or effects are not disclosed in any publicly known document.

The aforementioned international application: WO 2004/024204 discloses ahoneycomb filter including exhaust gas introduction cells 102 eachhaving an octagonal cross sectional shape and exhaust gas emission cells101 each having a rectangular cross sectional shape as shown in FIG.19B. WO 2004/024294 describes that an increase in the cross sectionalarea of the exhaust gas introduction cells 102 enables wide and thinaccumulation of PMs, and thus the transitional pressure loss can bereduced. However, for achieving the present invention in view of WO2004/024294, some of the exhaust gas emission cells 101 each having asmaller cross sectional area need to be changed to the exhaust gasintroduction cells 102, and some of the exhaust gas introduction cells102 each having a larger cross sectional area need to be changed to theexhaust gas emission cells 101. Such changes deny the inventive conceptof the invention of WO 2004/024294, i.e. increasing the cross sectionalarea of the exhaust gas introduction cells 102. Thus, the presentinvention cannot be achieved in view of WO 2004/024294 as the closestbackground art document.

Also, as explained earlier based on FIGS. 20A and 20B, U.S. Pat. No.4,417,908 discloses a honeycomb filter which can reduce the transitionalpressure loss by increasing the number of exhaust gas introduction cellshaving the same cross sectional area to increase the total area of theexhaust gas introduction cells so that PMs are allowed to widely andthinly accumulate.

However, for achieving the present invention in view of U.S. Pat. No.4,417,908, sore of the exhaust gas introduction cells need to be changedto cells having a smaller cross sectional area. Such a change reducesthe cross sectional area of the exhaust gas introduction cells, and thusdenies the inventive concept of U.S. Pat. No. 4,417,908. Hence, thepresent invention is not achieved in view of U.S. Pat. No. 4,417,908 asthe closest background art document.

Those background art documents deny the present invention, and thus thepresent invention cannot be achieved in view of the background arts.

The following will explain the details of the effects of the embodimentsof the present invention by exemplifying an embodiment.

FIGS. 6A to 6C are each an enlarged end face view illustrating anenlarged image of a part of an end face of the honeycomb filteraccording to one embodiment of the present invention.

As shown in FIG. 6A, in a honeycomb filter 20, each exhaust gas emissioncell 11 having an open end at an exhaust gas emission side and a pluggedend at an exhaust gas introduction side is adjacently surrounded fullyby first exhaust gas introduction cells 12 and second exhaust gasintroduction cells 14 each having an open end at the exhaust gasintroduction side and a plugged end at the exhaust gas emission sideacross porous cell walls 13.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cell 11 has an octagonal cross sectionthat is the same as or similar to that of the exhaust gas introductioncell 102 shown in FIG. 19B, the first exhaust gas introduction cell 12has a square cross section, and the second exhaust gas introduction cell14 has an octagonal square section that is the same as that of theexhaust gas emission cell 11. The second exhaust gas introduction cell14 has a larger cross sectional area than the first exhaust gasintroduction cell 12, and the cross sectional area is the same as theexhaust gas emission cell 11. That is, the second exhaust gasintroduction cell 14 has the same cross sectional area as the exhaustgas emission cell 11, and the exhaust gas emission cell 11 has a largercross sectional area than the first exhaust gas introduction cell 12.Thus, the resistance caused by flowing of exhaust gas through theexhaust gas emission cells 11 and the resistance caused by outflow ofexhaust gas to outside the filter are reduced to low levels, and therebythe pressure loss can be reduced to a low level.

Moreover, a side 12 a facing the exhaust gas emission cell 11 among thesides forming the cross sectional shape of the first exhaust gasintroduction cell 12 is longer than a side 14 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell 14.

Exhaust gas flowing toward the honeycomb filter 20 flows into the firstexhaust gas introduction cells 12 each having an open end at the exhaustgas introduction side and the second exhaust gas introduction cells 14each having an open end at the exhaust gas introduction side. Theexhaust gas flows in the filter in order from a part allowing easierflow and then evenly flows in the entire filter. In the honeycomb filteraccording to the embodiments of the present invention, the length (Ls)of the side 12 a of the first exhaust gas introduction cell 12 is largerthan the length (Lo) of the side 14 a of the second exhaust gasintroduction cell 14. Thus, the surface area of a cell wall 13 aseparating the exhaust gas emission cell 11 and the first exhaust gasintroduction cell 12 is larger than the surface area of a cell wall 13 bseparating the exhaust gas emission cell 11 and the second exhaust gasintroduction cell 14, leading to easier exhaust gas passage through thecell wall 13 a. Consequently, PMs accumulate on the cell walls 13 a atan initial stage.

As described above, both of the flow-through resistance of the exhaustgas emission cells and the outflow resistance upon emission of exhaustgas from the honeycomb filter can be reduced. Thus, the initial pressureloss before accumulation of PMs can be reduced.

The relation between the length of a side forming a cell and the surfacearea mentioned above is attributed to the following reasons.

The surface area of the cell wall 13 a separating the exhaust gasemission cell 11 and the first exhaust gas introduction cell 12corresponds to the surface area of the inner wall of the first exhaustgas introduction cell 12. The surface area of the inner wall of thefirst exhaust gas introduction cell 12 is expressed as Ls×Le, where Lerepresents an effective length of the filter excluding the length of theplugged portions at the introduction side from the exhaust gasintroduction end and at the emission side from the exhaust gas emissionend (see FIG. 2B). Similarly, the surface area of the cell wall 13 bseparating the exhaust gas emission cell 11 and the second exhaust gasintroduction cell 14 corresponds to the surface area of the inner wallof the second exhaust gas introduction cell 14. The surface area of theinner wall of the second exhaust gas introduction cell 14 is expressedas Lo×Le, where Le represents an effective length of the filterexcluding the length of the plugged portions at the introduction sidefrom the exhaust gas introduction end and at the emission side from theexhaust gas emission end. The effective length of the filter is definedas a length measured from the tip of the plugged portion in FIG. 2B.

Thus, if the length (Ls) of the side 12 a is larger than the length (Lo)of the side 14 a, the surface area value of Ls×Le is relatively largerthan the surface area value of Lo×Le. Namely, the length of the side isequivalent to the surface area. Thus, if the length (Ls) of the side 12a of the first exhaust gas introduction cell 12 is larger than thelength (Lo) of the side 14 a of the second exhaust gas introduction cell14, the surface area of the cell wall 13 a separating the exhaust gasemission cell 11 and the first exhaust gas introduction cell 12 islarger than the surface area of the cell wall 13 b separating theexhaust gas emission cell 11 and the second exhaust gas introductioncell 14.

In FIGS. 6A to 6C, insertions relating to the effects are only partiallyillustrated.

Next, as shown in FIG. 6B, when a certain amount of PMs are accumulatedon the cell walls 13 a corresponding to the inner walls of the firstexhaust gas introduction cells 12, an accumulated layer of the PMs getsthick due to the small cross sectional area of the first exhaust gasintroduction cells 12. Consequently, resistance due to the accumulationof PMs increases to make the passage of exhaust gas through the cellwalls 13 a difficult. Under such conditions, exhaust gas turns to passthrough the cell walls 13 b separating the exhaust gas emission cells 11and the second exhaust gas introduction cells 14 (switching of the mainchannel). Then, PMs are also accumulated on the surfaces of the cellwalls 13 corresponding to the surfaces of the inner walls of the secondexhaust gas introduction cells 14.

Subsequently, since exhaust gas can considerably freely pass through thecell walls, exhaust gas passes through inside the cell walls 13 cseparating the first exhaust gas introduction cells 12 and the secondexhaust gas introduction cells 14 to flow into the exhaust gas emissioncells 11 as shown in FIG. 6C. In this situation, exhaust gas enters thecell walls 13 c from the first exhaust gas introduction cells 12 as wellas from the second exhaust gas introduction cells 14.

As described above, PMs accumulate on the entire surfaces of the cellwalls 13 a and 13 c around the first exhaust gas introduction cells 12corresponding the inner walls of the first exhaust gas introductioncells 12, and gradually accumulate rather more widely and thinly in alarger amount on the entire surfaces of the cell walls 13 b and 13 caround the second exhaust gas introduction cells 14 corresponding to theinner walls of the second exhaust gas introduction cells 14. The firstexhaust gas introduction cells 12 each have a smaller cross sectionalarea than each second exhaust gas introduction cell 14. Thus, in thefirst exhaust gas introduction cells 12, PMs accumulate in a thick layerwhere the passage resistance is high. For this reason, the introducedexhaust gas flows more easily into the second exhaust gas introductioncells 14 than into the first exhaust gas introduction cells 12 at anearly stage (aforementioned switching of the main channel of exhaustgas), causing the aforementioned shift of the PM accumulation.Consequently, PMs accumulate more on the entire surfaces of the cellwalls 13 b and 13 c around the second exhaust gas introduction cells 14corresponding to the inner walls of the second exhaust gas introductioncells 14 rather than the surfaces of the cell walls 13 a and 13 c aroundthe first exhaust gas introduction cells 12 corresponding to the innerwalls of the first exhaust gas introduction cells 12. It is thuspossible to make use of the entire surfaces of the cell walls 13 b and13 c around the second exhaust gas introduction cells 14 correspondingto the inner walls of the second exhaust gas introduction cells 14 foraccumulation of PMs earlier. Since the surface area of the cell walls 13b and 13 c around the second exhaust gas introduction cells 14corresponding to the inner walls of the second exhaust gas introductioncells 14 is larger than the surface area of the cell walls 13 a and 13 caround the first exhaust gas introduction cells 12 corresponding to theinner walls of the first exhaust gas introduction cells 12, when PMsaccumulate on the entire peripheries of the cell walls 13 b and 13 csurrounding the second exhaust gas introduction cells 14, theaccumulated layer is thin. Thus, the pressure loss due to exhaust gasincreases at a low rate even after accumulation of PMs. Hence, asurprisingly excellent effect of maintaining the pressure loss at a lowlevel can be achieved even though the amount of accumulated PMsincreases.

As a result, vehicles carrying the honeycomb filter according to theembodiments of the present invention do not cause a disadvantageousphenomenon for driving that are derived from an increase in the pressureloss throughout the use area, and also have good fuel economy.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of an exhaust gas emissioncell and a first exhaust gas introduction cell which have polygonalcross sectional shapes and are adjacent to each other, a side, among thesides forming the cross sectional shape of the exhaust gas emissioncell, which is adjacent to the first exhaust gas introduction cellacross the cell wall in a manner facing the first exhaust gasintroduction cell is parallel to a side, among the sides forming thecross sectional shape of the first exhaust gas introduction cell, whichis adjacent to the exhaust gas emission cell across the cell wall in amanner facing the exhaust gas emission cell.

This indicates that the thickness is uniform at any part of the wallsseparating the exhaust gas emission cells and the first exhaust gasintroduction cells. Thus, if is possible to achieve high fracturestrength of the filter, easy passage of exhaust gas, and uniformaccumulation of PMs, so that the pressure loss is reduced.

In the case where the vertex portions of the polygonal cross section areformed by curved lines, the curve portions are not considered as sidesbecause naturally such portions do not form parallel lines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of an exhaust gas emissioncell and a second exhaust gas introduction cell which is adjacent to theexhaust gas emission cell across a cell wall, in the case where thecells nave polygonal cross sections, a side, among the sides forming thecross sectional shape of the exhaust gas emission cell, which isadjacent to the second exhaust gas introduction cell across the cellwall in a manner facing the second exhaust gas introduction cell isparallel to a side, among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell, which is adjacent to theexhaust gas emission cell across the cell wall in a manner facing theexhaust gas emission cell.

This indicates that the thickness is uniform at any part of the wallsseparating the exhaust gas emission cells and the respective secondexhaust gas introduction cells. Thus, it is possible to achieve highfracture strength of the filter, easy passage of exhaust gas, anduniform accumulation of PMs so that the pressure loss is reduced.

Meanwhile, in the case where the vertex portions of the polygonal crosssectional shape are formed by curved lines, the curve portions are notconsidered as sides because naturally such portions do not form parallellines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of a first exhaust gasintroduction cell and a second exhaust gas introduction cell which isadjacent to the first exhaust gas introduction cell across a cell wall,in the case where the cells have polygonal cross sectional shapes, aside, among the sides forming the cross sectional shape of the firstexhaust gas introduction cell, which is adjacent to the second exhaustgas introduction cell across the cell wall in a manner facing the secondexhaust gas introduction cell, is parallel to a side, among the sidesforming, the cross sectional shape of the second exhaust gasintroduction cell, which is adjacent to the first exhaust gasintroduction cell across the cell wall in a manner facing the firstexhaust gas introduction cell.

This indicates that the thickness is uniform at any part of the wallsseparating the first exhaust gas introduction cells and the secondexhaust gas introduction cells. Thus, it is possible to achieve highfracture strength of the honeycomb filter, easy exhaust gas passagethrough the walls from the second exhaust gas introduction cells to theexhaust gas emission cells, and wide, thin and uniform accumulation ofPMs on the inner cell walls of the second exhaust gas introductioncells, so that low pressure loss can be achieved after accumulation ofPMs.

Meanwhile, in the case where the vertex portions of the polygonal crosssection are formed by curved lines, the curve portions are notconsidered as sides because naturally such portions do not form parallellines.

Provided that, in the cross section perpendicular to the longitudinaldirection, of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put if the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter according to the embodiments of the presentinvention preferably has the following structure. In the case where theexhaust gas emission cell, the first exhaust gas introduction cell, andthe second exhaust gas introduction cell, which are adjacent to oneanother across cell walls, each have a polygonal cross sectional shapein the cross section perpendicular to the longitudinal direction of thecells,

(a) a side, among the sides forming the cross sectional shape of theexhaust gas emission cell, which is adjacent to the first exhaust gasintroduction cell across the cell wall in a manner facing the firstexhaust gas introduction cell, is parallel to a side, among the sidesforming the cross sectional shape of the first exhaust gas introductioncell, which is adjacent to the exhaust gas emission cell across the cellwall in a manner facing the exhaust gas emission cell,

(b) a side, among the sides forming the cross sectional shape of theexhaust gas emission cell, which is adjacent to the second exhaust gasintroduction cell across the cell wall in a manner facing the secondexhaust gas introduction cell, is parallel to a side, among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, which is adjacent to the exhaust gas emission cell across the cellwall in a manner facing the exhaust gas emission cell, and

(c) a side, among the sides forming the cross sectional shape of thefirst exhaust gas introduction cell, which is adjacent to the secondexhaust gas introduction cell across the cell wall in a manner facingthe second exhaust gas introduction cell, is parallel to a side, amongthe sides forming the cross sectional shape of the second exhaust gasintroduction cell, which is adjacent to the first exhaust gasintroduction cell across the cell wall in a manner facing the firstexhaust gas introduction cell.

Moreover, in the case where the first exhaust gas introduction cell, thesecond exhaust gas introduction cell, and the exhaust gas emission celleach have a polygonal cross sectional shape in the honeycomb filteraccording to the embodiments of the present invention, the distancebetween the parallel sides in the above condition (a), the distancebetween the parallel sides in the above condition (b), and the distancebetween the parallel sides in the above condition (c) are preferably thesame in the structure simultaneously satisfying the conditions (a), (b),and (c). Here, the distance between the sides is defined as follows: ahypothetical perpendicular line is given from an arbitrary point P inone side to a point Q in the other side, and the distance between thepoint P and the point Q is defined as the distance between the parallelsides.

The honeycomb filter having the above structure has the highest fracturestrength, has excellent thermal shock resistance upon regeneration, canbest reduce the pressure loss regardless of the presence or absence ofaccumulated PMs, and can avoid thermal shock damage that occurs uponregeneration of PMs.

Meanwhile, in the case where the vertex portions of the polygonal crosssection are formed by curved lines, the curve portions are notconsidered as sides because naturally such portions do not form parallellines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter of the embodiments of the present invention ispreferably need to purify PMs an exhaust gas discharged from internalcombustion engines of automobiles. The honeycomb filter can reduce bothof the initial pressure loss before accumulation of PMs and thetransitional pressure loss caused by accumulation of PMs in the filter,and thereby the fuel economy of the engine can be enhanced.

The honeycomb filter of the embodiments of the present invention is mostsuitably used in automobiles whose internal combustion engines arediesel engines. The amount of PMs (soot) discharged from a diesel engineis larger than that from a gasoline engine. Thus, a demand for reducingthe transitional pressure loss caused by accumulation of PMs (soot) inthe filter is higher for diesel engines than for gasoline engines.

In the case of using the honeycomb each filter of the embodiments of thepresent invention to purify PMs in exhaust gas discharged from internalcombustion engines of automobiles, the honeycomb filter of theembodiments of the present invention is fixed inside an exhaust tube viaa holding material.

In the honeycomb filter according to the embodiments of the presentinvention, in the cross section perpendicular to the longitudinaldirection of the cells, each exhaust gas emission cell and each exhaustgas introduction cell preferably have a polygonal cross section, and aside forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, a sideforming the cross sectional shape of each second exhaust gasintroduction cell faces one of the exhaust gas emission cells, and thelength of the side forming the cross sectional shape of the secondexhaust gas introduction cell is not more than 0.8 times the length ofthe side forming the cross sectional shape of the first exhaust gasintroduction cell.

The honeycomb filter having the above structure enables easier passageof exhaust gas through the cell walls separating the exhaust gasemission cells and the first exhaust gas introduction cells, effectivesuppression of the initial pressure loss, and prevention of an increasein the rate of increase of the pressure loss after accumulation of PMs.

If the ratio of the length of the side of the second exhaust gasintroduction cell to the length of the side of the first exhaust gasintroduction cell exceeds 0.8, the two sides do not have a bigdifference in length. Consequently, the initial pressure loss is hardlysuppressed.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, preferably, the exhaust gas emission cells are eachoctagonal, the first exhaust gas introduction cells are each square, andthe second exhaust gas introduction cells are each octagonal.

The honeycomb filter having the above structure has the same shape asthe honeycomb filter in FIGS. 6A to 6C that is described concerning theeffects thereof. Thus, the honeycomb filter can effectively suppress theinitial pressure loss, have a large surface area for allowing PMs toaccumulate thereon, and can maintain the pressure loss at a low level.

In the honeycomb filter according to the embodiments of the presentinvention, in the cross section perpendicular to the longitudinaldirection of the cells,

preferably, the cross sectional area of each second exhaust gasintroduction cell is equal in size to the cross sectional area of eachexhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

The honeycomb structure having the above structure can providedifference between the resistance caused upon flowing of exhaust gasthrough the first exhaust gas introduction cells and the resistancecaused upon flowing of exhaust gas through the second exhaust gasintroduction cells, thereby enabling effective suppression of thepressure loss.

If the cross sectional area of the first exhaust gas introduction cellsis less than 20% the size of the cross sectional area of the secondexhaust gas introduction cells, the cross sectional area of the firstexhaust gas introduction cells is too small. Consequently, highflow-through resistance occurs upon flowing of exhaust gas through thefirst exhaust gas introduction cells so that the pressure loss tends tobe high. If the cross sectional area of the first exhaust gasintroduction cells is more than 50% the size of the cross sectional areaof the second exhaust gas introduction cells, the difference in theflow-through resistance of the first exhaust gas introduction cells andthe flow-through resistance of the second exhaust gas introduction cellsis small. Thus, the pressure loss is hardly reduced.

In the honeycomb filter according to the embodiments of the presentinvention, the cell walls defining rims of the cells preferably have auniform this tress in any part of the honeycomb filter.

The honeycomb filter having this structure can exert the aforementionedeffects through its entire body.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

preferably the exhaust gas emission cells have octagonal cross sections,the first exhaust gas introduction cells nave square cross sections, andthe second exhaust gas introduction cells have octagonal cross sections,

the cross sectional shape of each second exhaust gas introduction cellis congruent with the cross sectional shape of each exhaust gas emissioncell, and

the exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells are arranged in thefollowing manner:

the exhaust gas emission cells are each surrounded by alternatelyarranged four pieces of the first exhaust gas introduction cells andfour pieces of the second exhaust gas introduction cells across theporous cell walls;

provided that hypothetical segments connecting geometric centers ofgravity of the octagonal cross sections of the four second exhaust gasintroduction cells surrounding the exhaust gas emission cell are given,an intersection of the two segments crossing a shape region includingthe cross sectional shape of the exhaust gas emission cell is identicalwith a geometric center of gravity of the octagonal cross section of theexhaust gas emission cell; and

the four segments not crossing the shape region including the crosssectional shape of the exhaust gas emission cell form a square, andmidpoints of the respective sides of the square are identical withgeometric centers of gravity of the square cross sections of the fourfirst exhaust gas introduction cells surrounding the exhaust gasemission cell, and

a side facing the first exhaust gas introduction cell across a cell wallamong the sides forming the cross sectional shape of the exhaust gasemission cell is parallel to a side facing the exhaust gas emission cellacross the cell wall among the sides forming the cross sectional shapeof the first exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cellwall among the sides forming the cross sectional shape of the exhaustgas emission cell is parallel to a side facing the exhaust gas emissioncell across the cell wall among the sides forming the cross sectionalshape of the second exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cellwall among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is parallel to a side facing the firstexhaust gas introduction cell across the cell wall among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, and distances between the parallel sides are the same.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, the exhaust gas emission cells, the first exhaust gasintroduction cells, and the second exhaust gas introduction cells allpreferably have a square cross section.

Even in the case where the first exhaust gas introduction cells and thesecond exhaust gas introduction cells all have a square cross section,relations concerning the size, position, or the like of the exhaust gasemission cells, the first exhaust gas introduction cells, and the secondexhaust gas introduction cells are different. For example, since thecross sectional area of each first exhaust gas introduction cell issmaller than the cross sectional area of each exhaust gas emission cell,the honeycomb filter of the embodiments of the present invention isdifferent from the honeycomb filter 110 (see FIGS. 20A and 20B) in theaforementioned background art, and can exert the aforementioned effectsof the embodiments of the present invention.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, preferably the cross sectional area of each second exhaustgas introduction cell is equal in size to the cross sectional area ofeach exhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

The honeycomb filter having the above structure can provide differencebetween the resistance caused upon flowing of exhaust gas through thefirst exhaust gas introduction cells and the resistance caused uponflowing of exhaust gas through the second exhaust gas introductioncells, thereby enabling affective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cellsis less than 20% the size of the cross sectional area of the secondexhaust gas introduction cells, the cross sectional area of the firstexhaust gas introduction cells is too small. Consequently, highflow-through resistance occurs upon flowing of exhaust gas through thefirst exhaust gas introduction cells so that the pressure loss tends tobe high. If the cross sectional area of the first exhaust gasintroduction cells is more than 50% the size of the cross sectional areaof the second exhaust gas introduction cells, the difference inflow-through resistance between the first exhaust, gas introductioncells and the second exhaust gas introduction cells is small. Thus, thepressure loss is hardly reduced.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

preferably, the exhaust gas emission cells have a square cross section,the first exhaust gas introduction cells have a square cross section,and the second, exhaust gas introduction cells have a square crosssection,

the cross sectional shape of each second exhaust gas introduction cellis congruent with the cross sectional shape of each exhaust gas emissioncell,

the exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells are arranged in thefollowing manner:

the exhaust gas emission cells are each surrounded by alternatelyarranged four pieces of the first exhaust gas introduction cells andfour pieces of the second exhaust gas introduction cells across theporous cell walls;

provided that hypothetical segments connecting geometric centers ofgravity of the square cross sections of the four second exhaust gasintroduction cells surrounding the exhaust gas emission cell are given,an intersection of the two segments crossing a shape region includingthe cross sectional shape of the exhaust gas emission cell is identicalwith a geometric center of gravity of the square cross section of theexhaust gas emission cell; and

the four segments not crossing the shape region including the crosssectional shape of the exhaust gas emission cell form a square, andmidpoints of the sides of the square are identical with geometriccenters of gravity of the square cross sections of the four firstexhaust gas introduction cells surrounding the exhaust gas emissioncell, and

a side facing the first exhaust gas introduction cell across a cell wallamong the sides forming the cross sectional shape of the exhaust gasemission cell is parallel to a side facing the exhaust gas emission cellacross the cell wall among the sides forming the cross sectional shapeof the first exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cellwall among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is parallel to a side facing the firstexhaust gas introduction cell across the cell wall among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, and distances between the parallel sides are the same.

In the honeycomb filter of the embodiments of the present invention, inthe cross section perpendicular to the longitudinal direction of theaforementioned cells, preferably the vertex portions of the polygonalcells are formed by curved lines.

The honeycomb filter having cells with vertex portions formed by carvedlines according to the above structure are not susceptible toconcentration of stress derived from heat or the like in the cornerportions of the cells. Thus, cracks hardly occur.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

the exhaust gas emission cells, the first exhaust, gas introductioncells, and the second exhaust gas introduction cells are preferablypoint-symmetrical polygons each having not move than eight sides.

The cells each having a point-symmetrical polygonal shape with not morethan eight sides can reduce the resistance caused upon flowing ofexhaust gas through the cells, and thus can further reduce the pressureloss.

In the honeycomb filter according to the embodiments of the presentinvention, preferably, the exhaust gas introduction cells include firstexhaust gas introduction cells and second exhaust gas introduction cellseach having a larger cross sectional area than each first exhaust gasintroduction cell in a direction perpendicular to the longitudinaldirection of the cells;

each exhaust gas emission cell has an equal or larger cross sectionalarea than each second exhaust gas introduction cell in a directionperpendicular to the longitudinal direction of the cells;

the exhaust gas emission cells and the exhaust gas introduction cellsare each in a shape formed by curved lines in the directionperpendicular to the longitudinal direction of the cells; and thethickness of the cell walls separating the first exhaust gasintroduction cells and the respective exhaust gas emission cells issmaller than the thickness of the cell walls separating the secondexhaust gas introduction cells and the respective exhaust gas emissioncells.

In the honeycomb filter according to the embodiments of the presentinvention, in a case where the thickness of the cell walls separatingthe first exhaust gas introduction cells and the respective exhaust gasemission cells is smaller than the thickness of the cell wallsseparating the second exhaust gas introduction cells and the respectiveexhaust gas emission cells, exhaust gas easily passes through the cellwalls separating the first exhaust gas introduction cells and therespective exhaust gas emission cells at an early stage. Afteraccumulation of a certain amount of PMs, exhaust gas is likely to passthrough the cell walls separating the second exhaust gas introductioncells and the respective exhaust gas emission cells. The second exhaustgas introduction cells have a larger cross sectional area than the firstexhaust gas introduction cells, and the exhaust gas emission cells havean equal or larger cross sectional area than the respective secondexhaust gas introduction cells. For this reason, the above effects ofthe embodiments of the present invention are exerted.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

preferably the exhaust gas introduction cells and the exhaust gasemission cells are each in a shape formed by curved lines, and

the thickness of the cell walls separating the first exhaust gasintroduction cells and the respective exhaust gas emission cells is 40to 75% the thickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells.

In the honeycomb filter of the embodiments or the present invention, ifthe thickness of the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells is 40 to 75% thethickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells, exhaust gaseasily passes through the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells at an initialstage. Then, after accumulation of a certain amount of PMs, exhaust gaspasses through the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells. Moreover, thesecond exhaust gas introduction cells each have a larger cross sectionalarea than each first exhaust gas introduction cell, and each exhaust gasemission cell has an equal or larger cross sectional area than eachsecond exhaust gas introduction cell. Thus, the aforementioned effectsof the embodiments of the present invention can be exerted.

If the thickness of the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells is less than 40%the thickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells, the cell wallsseparating the first exhaust gas introduction cells and the exhaust gasemission cells need to be extremely thin. Consequently, the honeycombfilter has low mechanical strength. If the thickness of the cell wallsseparating the first exhaust gas introduction cells and the exhaust gasemission cells is more than 75% the thickness of the cell wallsseparating the second exhaust gas introduction cells and the exhaust gasemission cells, the former cells and latter cells do not have a bigdifference in the thickness. Consequently, the aforementioned pressureloss reduction effect may not be obtained.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

the exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells are preferablyround.

In the honeycomb filter having the above structure, if the crosssectional shapes of the exhaust gas emission cells, first exhaust gasintroduction cells, and the second exhaust gas introduction cells areall round, the effects of the embodiments of the present invention canbe exerted.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

the exhaust gas emission cells and the second exhaust gas introductioncells each preferably have a convex square cross section formed by fouroutwardly curved lines, whereas the first exhaust gas introduction cellseach preferably have a concave square cross section formed by fourinwardly curved lines.

The term “convex square” herein refers to a shape formed by fouroutwardly curved lines in the same length. The shape seems to be asquare having sides expanding outwardly from the geometrical center ofgravity. The term “concave square” herein refers to a shape formed byfour inwardly curved lines in the same length. The shape seems to be asquare having sides bending toward the geometrical center of gravity.

The exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells in this honeycombfilter having this structure have the aforementioned structures. Thus,the exhaust gas emission cells each have a larger cross sectional areathan each first exhaust gas introduction cells, and thereby providing ahoneycomb filter having the relations concerning the sizes of theexhaust gas emission cells, first exhaust gas introduction cells, andsecond exhaust gas introduction cells according to the embodiments ofthe present invention. Consequently, the effects of the embodiments ofthe present invention can be exerted.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal direction,of the cells,

the cross sectional area of each second exhaust gas introduction cell ispreferably equal in size to the cross sectional area of each exhaust gasemission cell, and

the cross sectional area of each first exhaust gas introduction cell ispreferably 20 to 50% the size of the cross sectional area of each secondexhaust gas introduction cell.

The honeycomb filter having the above structure can provide differencebetween the resistances caused upon flowing of exhaust gas through thefirst exhaust gas introduction cells and the resistance caused uponflowing of exhaust gas through the second exhaust gas introductioncells, thereby enabling effective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cellsis less than 20% the size of the cross sectional area of the secondexhaust gas introduction cells, the cross sectional area of the firstexhaust gas introduction cells is too small. Consequently, highflow-through resistance occurs upon flowing of exhaust gas through thefirst exhaust gas introduction cells so that the pressure loss tends tobe high. If the cross sectional area of the first exhaust gasintroduction cells is more than 50% the size of the cross sectional areaof the second exhaust gas introduction cells, the difference between theflow-through resistance of the first exhaust gas introduction cells andthe flow-through resistance of the second exhaust gas introduction cellsis small. Thus, the pressure loss is hardly reduced.

In the honeycomb filter of the embodiments of the present invention, theexhaust gas introduction cells preferably consist only of the firstexhaust gas introduction cells and the second exhaust gas introductioncells each having a larger cross sectional area than each first exhaustgas introduction cell in a direction perpendicular too the longitudinaldirection of the cells.

This is because a smaller number of first exhaust gas introduction cellseach having a smaller cross sectional area than each second exhaust gasintroduction cell provides a larger effective introduction cell area,thereby allowing PMs to be thinly and widely accumulated.

The honeycomb filter of the embodiments of the present inventionpreferably includes a plurality of honeycomb fired bodies combined withone another by adhesive layers residing therebetween, the honeycombfired bodies each having the exhaust gas emission cells, the firstexhaust gas introduction cells, and the second exhaust gas introductioncells, and each having an outer wall on the periphery thereof.

With regard to the structure formed by combining a plurality ofhoneycomb fired bodies to one another with adhesive layers residingtherebetween, since the cells forming one honeycomb fired body have thestructures according to the embodiments of the present invention, anaggregate of the honeycomb fired bodies can exert the effects of theembodiments of the present invention.

Moreover, the honeycomb fired bodies each having a smaller volumeenables to reduce the thermal stress that is generated upon productionand usage thereof, preventing damage such as cracks.

In the honeycomb filter of the embodiments of the present invention, theouter wall of the honeycomb fired body and the exhaust gas introductioncell adjacent to the outer wall have the following three patterns ofshapes:

(1) as illustrated in FIGS. 12B and 14A, the thickness of the outer wallis not uniform, and the first exhaust gas introduction cell and theexhaust gas emission cell adjacent to the outer wall have the sameshapes as the first exhaust gas introduction cell and the exhaust gasemission cell not adjacent to the outer wall, respectively;

(2) as illustrated in FIGS. 2A and 14B, the thickness of the outer wallis uniform, the first exhaust gas introduction cell adjacent to theouter wall has the same shape as the first exhaust gas introduction cellnot adjacent to the outer wall, and the exhaust gas emission celladjacent to the outer wall has a shape partially deformed, compared tothe cross sectional shape of the exhaust gas emission cell not adjacentto the outer wall, in accordance with the line along the inner wall,which forms the outer wall, in the exhaust gas emission cell adjacent tothe outer wall; and

(3) as illustrated in FIGS. 13B and 14C, the thickness of the outer wallis uniform in accordance with the cross sectional shapes of the firstexhaust gas introduction cell and the exhaust gas emission cell adjacentto the outer wall, and the first exhaust gas introduction cell and theexhaust gas emission cell adjacent to the outer wall have the sameshapes as the first exhaust gas introduction cell and the exhaust gasemission cell not adjacent to the outer wall, respectively, which meansthat the outer wall is bending in accordance with the cross sectionalshapes of the first exhaust gas introduction cell and the exhaust gasemission cell adjacent to the outer wall.

In the honeycomb filter of the embodiments of the present invention,preferably, the outer wall has corner portions and in the exhaust gasintroduction cells and the exhaust gas emission cells adjacent to theouter wall, a side, which contacts the outer wall, is straight andparallel to a side corresponding to an outer periphery of the outerwall.

The pattern (2) in the above description of three patterns applies tothe honeycomb filter of such an embodiment.

The aforementioned structure enables to enhance the strength of thehoneycomb fired body by the outer wall but also suppress the partialvariation of the volume ratio of the exhaust emission cells and theexhaust gas introduction cells in the honeycomb fired body. As a result,the flow of exhaust gas becomes more uniform, lowering the pressureloss.

According to she honeycomb filter of the embodiments of the presentinvention, the aforementioned honeycomb filter includes honeycomb firedbodies, and the honeycomb fired body preferably includes silicon carbideor silicon-containing silicon carbide.

The silicon carbide and silicon-containing silicon carbide are materialsexcellent in heat resistance. Thus, the honeycomb filter is better inheat resistance.

In the honeycomb filter of the embodiments of the present invention, thethickened of the cell wall of the honeycomb filter is preferably 0.10 to0.46 mm.

The cell wall having such a thickness is enough thick to capture PMs inthe gas and effectively suppresses an increase of the pressure loss. Asa result, the honeycomb filter of the embodiments of the presentinvention can sufficiently exert the effects of the embodiments of thepresent invention.

If the thickness is less than 0.10 mm, the cell wall is too thin, sothat the mechanical strength of the honeycomb filter is lowered. If thethickness is more than 0.46 mm, the cell wall is too thick, so that thepressure loss upon passage of exhaust gas through the cell wall becomesgreater.

In the honeycomb filter of the embodiments of the present invention, thecell walls have pores having an average pore diameter of 8 to 25 μm. Thepore diameter and the porosity are measured by a mercury injectionmethod under conditions of the contact angle of 130 degrees and thesurface tension of 485 mN/m.

If the cell walls have pores having an average pore diameter of lessthan 8 μm, the pressure loss increases after an oxidation catalyst issupported. If the cell walls have pores having an average pore diameterof more than 25 μm, the pore diameter is too large so that the effect ofimproving PM capturing efficiency decreases.

In the honeycomb filter of the embodiments of the present invention, thecell walls preferably have a porosity of 40 to 70%.

If the cell walls have a porosity of 40 to 70%, an increase in thepressure loss derived from supporting of the catalyst can be suppressed.Also, the cell walls can favorably capture PMs in exhaust gas. Moreover,an increase in the pressure loss derived from the cell walls can besuppressed. Thus, it is possible to provide a honeycomb filter which hasa low initial pressure loss and tends not to suffer an increase in thepressure loss even after accumulation of PMs.

If the cell walls have a porosity of less than 40%, the porosity in thecell walls is too small. Thus, supporting of an oxidation catalyst tendsto increase the pressure loss. If the cell walls have a porosity of morethan 70%, the cell walls have low mechanical strength, and thus crackstend to occur during regeneration or the like.

The honeycomb filter of the embodiments of the present inventionpreferably has a peripheral coat layer formed on the periphery thereof.

The periphery coat layer functions to protect cells inside thereof frommechanical damage. Thus, a honeycomb filter having excellent mechanicalcharacteristics such as compression strength is obtained.

In the honeycomb filter of the embodiments of the present invention, inthe cross section perpendicular to the longitudinal direction of thecells forming the honeycomb filter,

preferably the first exhaust gas introduction cells, the second exhaustgas introduction cells, and the exhaust gas emission cells each have auniform cross sectional shape except for the plugged portion in adirection perpendicular to the longitudinal direction of the cellsthoroughly from the end at the exhaust gas introduction side to the endat the exhaust gas emission side, the cross sectional shape of the firstexhaust gas introduction cells is different from the cross sectionalshape of the second exhaust gas introduction cells, and the crosssectional shape of the exhaust gas emission cells is different from thecross sectional shape of the first exhaust gas introduction cells. Here,“different” means “not congruent,” but encompasses “similar.” In otherwords, if the cross sectional shapes are similar to each other, thecross sectional shapes are considered different from each other.

Each first exhaust gas introduction cell itself has a uniform crosssectional shape at any cross section thereof, each second exhaust gasintroduction cell itself has a uniform cross sectional shape at anycross section thereof, and each exhaust gas emission cell has a uniformcross sectional shape at any cross section thereof. Each first exhaustgas introduction cell has a different cross sectional shape from eachsecond exhaust gas introduction cell. Each exhaust gas emission cell hasa different cross sectional shape from each first exhaust gasintroduction cell.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of the cells, a cell unithaving a cell structure described below is two-dimensionally repeated,where the first exhaust gas introduction cells and the second exhaustgas introduction cells surrounding each exhaust gas emission cell in theunit are shared between adjacent cell units.

cell structure: each exhaust gas emission cell is adjacently surroundedfully by the exhaust gas introduction cells across the porous cellwalls, the exhaust gas introduction cells including first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga larger cross sectional area than each first exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells, and each exhaust gas emission cell has an equal or larger crosssectional area than each second exhaust gas introduction cell in adirection perpendicular to the longitudinal direction of the cells, and

the exhaust gas introduction cells and the exhaust gas emission cellshave the following feature A or B in the cross section perpendicular tothe longitudinal direction of the cells:

A: the exhaust gas introduction cells and the exhaust gas emission cellsare each polygonal, and

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cellar a sideforming the cross sectional shape of each second exhaust gasintroduction cell faces one of the exhaust gas emission cells, and theside of the first exhaust gas introduction cell is longer than the sideof the second exhaust gas introduction cell, or

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, and noneof the sides forming the cross sectional shape of each second exhaustgas introduction cell faces the exhaust gas emission cells;

B: the exhaust gas introduction cells and the exhaust gas emission cellsare each in a shape formed by a curved line, and

the thickness of the cell walls separating the first exhaust gasintroduction, cells and the exhaust gas emission cells is smaller thanthe thickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells.

Such a honeycomb filter is preferable because the two-dimensionalrepetition of the cell unit forms a filter having a large capacity. Thefilter has an outer wall, and the cell units naturally do not spreadoutside the outer wall. The cell units are cut out properly to be fitinto the shape of the outer wall.

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

The present invention is not limited to those embodiments, and may bemodified within a scope not changing the gist of the present invention.

First Embodiment

The following will discuss a honeycomb filter according to the firstembodiment of the present invention.

The honeycomb filter according to the first embodiment of the presentinvention includes a plurality of honeycomb fired bodies. Each honeycombfired body includes exhaust gas emission cells each having an open endat an exhaust gas emission side and a plugged end at an exhaust gasintroduction side, and exhaust gas introduction cells each having anopen end at the exhaust gas introduction side and a plugged end at theexhaust gas emission side, the exhaust gas introduction cells includingfirst exhaust gas introduction cells and second exhaust gas introductioncells, and has an outer wall on the periphery thereof. The honeycombfired bodies are combined with one another by adhesive layers residingtherebetween. An oxidation catalyst is supported inside the cell walls.

In terms of the cells other than the cells adjacent to the outer wall,the exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction cell, and eachexhaust gas emission cell has the same cross sectional area as eachsecond exhaust gas introduction cell. In other words, the exhaust gasemission cells have a larger average cross sectional area than theexhaust gas introduction cells in a direction perpendicular to thelongitudinal direction of the cells, and the total volume of the exhaustgas introduction cells is larger than the total volume of the exhaustgas emission cells.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells and the exhaust gas introductioncells are each polygonal, and a side facing one exhaust gas emissioncell among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is longer than a side facing one exhaustgas emission cell among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell.

The cells adjacent to the outer wall include the first exhaust gasintroduction cells and the exhaust gas emission cells arrangedalternately with the first exhaust gas introduction cells. Each exhaustgas emission cell has a larger cross sectional area than each firstexhaust gas introduction cell in a direction perpendicular to thelongitudinal direction of the cells.

Specifically, the arrangement of two first exhaust gas introductioncells, one second exhaust gas introduction cell, and one exhaust gasemission cell is two-dimensionally repeated. Here, since the crosssectional areas thereof have the above relationships, the exhaust gasemission cells have a larger average cross sectional area than theexhaust gas introduction cells, and the total volume of the exhaust gasintroduction cells is larger than the total volume of the exhaust gasemission cells.

The outer wall has corner portions. In the exhaust gas introductioncells and the exhaust gas emission cells adjacent to the outer wall, aside, which contacts the outer wall, is straight and parallel to a sidecorresponding to an outer periphery of the outer wall, and the exhaustgas emission cells adjacent to the outer wall each have a shapepartially deformed.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except for the pluggedportion, in a direction perpendicular to the longitudinal direction ofthe cells thoroughly from the end at the exhaust gas introduction sideto the end at the exhaust gas emission side.

Gamma alumina and platinum (Pt) are supported as an oxidation catalystinside the cell walls of the honeycomb filter according to the firstembodiment of the present invention. The oxidation catalyst is notlimited to a combination of gamma alumina and platinum (Pt).

The total amount of the oxidation catalyst, i.e., gamma alumina andplatinum, supported inside the cell walls of the honeycomb filteraccording to the first embodiment of the present invention, is 5 to 60g/L, and preferably 10 to 30 g/L. Herein, the amount of zeolitesupported inside the cell walls as an oxidation catalyst means theweight of an oxidation catalyst per liter of an apparent volume of thehoneycomb filter.

The apparent volume of the honeycomb filter includes the volume of theadhesive layer and coat layer.

FIG. 1 is a perspective view schematically illustrating one example ofthe honeycomb filter according to the first embodiment of the presentinvention.

FIG. 2A is a perspective view schematically illustrating one example ofthe honeycomb fired body forming the honeycomb filter shown in FIG. 1.FIG. 2B is an A-A line cross sectional view of the honeycomb fired bodyshown in FIG. 2A.

The honeycomb filter 20 shown in FIG. 1 includes a ceramic block 18formed of a plurality of honeycomb fired bodies 10 combined with anadhesive layer 15 residing therebetween, and has a periphery coat layer16 for preventing leakage of emission gas on the periphery of theceramic block 18. The periphery coat layer 16 may be optionally formed.

Such a honeycomb filter including combined honeycomb fired bodies isalso referred to as an aggregated honeycomb filter.

The honeycomb fired body 10 has a rectangular pillar shape, and isroundly cornered at the corner portions in end faces thereof as shown inFIG. 2A. This prevents thermal stress concentration at the cornerportions to thereby prevent occurrence of damages such as cracks. Thecorner portions each may be chamfered in a manner to have a shape formedby straight lines.

In the honeycomb filter 20 according to the first embodiment, theexhaust gas emission cells each have an open end at the exhaust gasemission side and a plugged end at the exhaust gas introduction side,and the exhaust gas introduction cells each have an open end at theexhaust gas introduction side and a plugged end at the exhaust gasemission side. The plug is preferably made of the same material as thematerial of the honeycomb fired body.

In the honeycomb fired body 10 shown in FIG. 2A and FIG. 2B, the exhaustgas emission cells 11 having an octagonal cross section are eachadjacently surrounded fully by the first exhaust gas introduction cells12 each having a square cross section and the second exhaust gasintroduction cells 14 each having an octagonal cross section acrossporous cell walls therebetween. The first exhaust gas introduction cells12 and the second exhaust gas introduction cells 14 are alternatelyarranged around each exhaust gee emission cell 11. Each second exhaustgas introduction cell 14 has a larger cross sectional area than eachfirst exhaust gas introduction cell 12, and each exhaust gas emissioncell 11 has the same cross sectional area as each second exhaust gasintroduction cell 14. An outer wall 17 is formed on the periphery of thehoneycomb fired body 10. Cells adjacent to the outer wall 17 includeexhaust gas emission cells 11A and 11B and first exhaust gasintroduction cells 12A.

Each second exhaust gas introduction cell 14 and exhaust gas emissioncell 11 name octagonal cross sections, and the cross sections arecongruent with each other.

In the exhaust gas emission cells 11A and 11B and the exhaust gasintroduction cells 12A adjacent to the outer wall 17 in the honeycombfilter 20 according to the present embodiment, a side, which contactsthe outer wall 17, is straight and parallel to a side corresponding toan outer periphery of the outer wall 17 in a manner that the thicknessof the outer wall 17 is uniform except for the corner portions in thecross section perpendicular to the longitudinal direction of the cells.

The cross sessions of the exhaust gas emission cells 11A adjacent to theouter wall 17 are partially cut so that the cross sectional shapes arechanged from octagons to hexagons. The cross sectional shape of thefirst exhaust gas introduction cells 12A may be in a partially cut shapebut is preferably congruent with the cross sectional shape of the firstexhaust gas introduction cells 12.

The cross sections of second exhaust gas introduction cells 11B at thecorner portions of the honeycomb fired body 10 have been changed fromoctagon 110B formed by curved lines. The chamfered portions 110B of theexhaust gas emission cells 11B illustrated in FIG. 2A are chamfered tobe curved but may be linearly chamfered.

The aforementioned structure enables to not only enhance the strength ofthe honeycomb fired body by the outer wall but also further reduce thepartial variation in the volume ratio between the exhaust gas emissioncells and the exhaust gas introduction cells in the honeycomb firedbody. Consequently, the uniform flow of exhaust gas is improved. In thevicinity of the outer wall, exhaust gas smoothly flows into the firstexhaust gas introduction cells 12 and the cell wall and outer walleffectively function as a filter, so rear the pressure loss is reduced.

In the honeycomb fired body 10, the exhaust gas emission cells 11A and11B and the exhaust gas introduction cells 2A are alternately arrangedalong the outer wall 17, and the exhaust gas emission cells 11 isorderly arranged, inside the cells along the outer wall 17 with thefirst exhaust gas introduction cells 12 and the second exhaust gasintroduction cells residing therebetween. As above, the exhaust gasemission cells 11, the first exhaust gas introduction cells 12, and thesecond exhaust gas introduction cells 14 are arranged in a significantlyordered manner.

Each exhaust gas emission cell 11 and each second exhaust gasintroduction cell 14 have the same octagonal shape. The octagon ispoint-symmetry with respect to the center of gravity. In the octagon,all hypotenuse sides (14 a in FIGS. 6A to 6C) have the same length, andall vertical or horizontal sides (14 b in FIGS. 6A to 6C) have the samelength. Moreover, four first sides (the hypotenuse sides) and foursecond sides (the vertical or horizontal sides) are alternatelyarranged. The first sides and the second sides form an angle of 135°.

The term “hypotenuse side” generally refers to the longest side that isopposite to the right angle in a right-angled triangle. However, forconvenience for explanation, a “hypotenuse side” herein refers to a sidesuch as the side 14 a and the side 11 b that has a certain angle, exceptfor 90° and 0°, to below-mentioned four hypothetical segments. Fordifferentiation from this term, the term “vertical or horizontal side”herein refers to a side such as the side 14 b and the side 11 a that isparallel to or vertical to the below-mentioned four hypotheticalsegments.

The hypothetical segments mentioned in the explanation of the terms“hypotenuse side” and “vertical or horizontal sides” refer to fourhypothetical segments (the four segments form a square) that do notcross the cross sectional figure of the exhaust gas emission cell 11,among hypothetical segments that connect geometric centers of gravity ofthe respective cross sectional figures of the four second exhaust gasintroduction cells arranged around the exhaust gas emission cell 11.

Each first exhaust gas introduction cell 12 has a square cross section.

In the cross section of the three kinds of cells which are adjacent toeach other, namely, the exhaust gas emission cell 11, the second exhaustgas introduction cell 14, and the first exhaust gas introduction cell12, the side 11 a facing the first exhaust gas introduction cell 12across the cell wall 13 among the sides of the octagonal exhaust gasemission cell 11 is parallel to the side 12 a facing the exhaust gasemission cell 11 across the cell wall 13 among the sides of the squarefirst exhaust gas introduction cell 12.

Also, the side 11 b facing the octagonal second exhaust gas introductioncell 14 across the cell wall 13 among the sides of the octagonal exhaustgas emission cells 11 is parallel to the side 14 a facing the exhaustgas emission cell 11 across the cell wall 13 among the sides of theoctagonal second exhaust gas introduction cell 14. Moreover, the side 12b facing the second exhaust gas introduction cell 14 across the cellwall 13 among the sides of the first exhaust gas introduction cell 12 isparallel to the side 14 b facing the first exhaust gas introduction cell12 across the cell wall 13 among the sides of the second exhaust gasintroduction cell 14. Furthermore, the distances between the parallelsides of all the above pairs are the same (see FIGS. 6A to 6C). That is,the distance between the parallel sides 11 a and 12 a, the distancebetween the parallel sides 11 b and 14 a, and the distance between theparallel sides 12 b and 14 b are the same.

Additionally, the exhaust gas emission cells 11, the first exhaust gasintroduction cells 12, and the second exhaust gas introduction cells 14are arranged in a manner satisfying the conditions below.

Among the hypothetical segments connecting the geometric centers ofgravity of the octagonal shapes of the four second exhaust gasintroduction cells 14 surrounding the exhaust gas emission cell 11, anintersection of the two segments crossing the octagonal shape region ofthe exhaust gas emission cell 11 is identical with the geometric centerof gravity of the octagonal cross section of the exhaust gas emissioncell 11.

Moreover, among the hypothetical segments connecting the geometriccenters of gravity of the octagonal shapes of the four second exhaustgas introduction cells 14, the four segments not crossing the octagonalshape region of the exhaust gas emission cell 11 forms a square, andmidpoints of the respective sides of the square are identical with thegeometric centers of gravity of the respective square shapes of the fourfirst exhaust gas introduction cells 12 surrounding the exhaust gasemission cell 11.

As described above, the octagonal exhaust gas emission cell 11 isadjacently surrounded by alternately arranged four pieces of the firstsquare exhaust gas introduction cells 12 and four pieces of the secondoctagonal exhaust gas introduction cells 14 across the cell walls 13 toform a single unit. The unit is two-dimensionally repeated, where thefirst exhaust gas introduction cells 12 and the second exhaust gasintroduction cells 14 in the unit are shared between adjacent cellunits, to form a honeycomb filter. Since the units share the firstexhaust gas introduction cells 12 and the second exhaust gasintroduction cells 14, the first exhaust gas introduction cell 12 andthe second exhaust gas introduction cell 14, that adjoin the exhaust gasemission cell 11 across the cell walls 13, also adjoin the exhaust gasemission cell 11 in the adjacent unit across the cell wall 13.

FIG. 7 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 7 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections shown in FIG. 7, repeated in the case where the secondexhaust gas introduction cells 14 and the exhaust gas emission cells 11are octagonal, the first exhaust gas introduction cells 12 are square inthe cross section of the cells, and the aforementioned conditions aresatisfied, and also illustrates how the first exhaust gas introductioncells 12 and the second exhaust gas introduction cells 14 are sharedbetween the cell units (cell structure).

A cell unit 1, a cell unit 2, and a cell unit 3 each have a structure inwhich the exhaust gas emission cell 11 is fully surrounded byalternately arranged four pieces of the first exhaust gas introductioncells 12 and four pieces of the second exhaust gas introduction cells 14across the cell wells 13 in a manner satisfying the aforementionedconditions. The cell unit 2 has the same structure as that of the cellunit 1. The cell unit 2 is adjacent to the cell unit 1 in the Xdirection while sharing one piece of the first exhaust gas introductioncell 12 and two pieces of the second exhaust gas introduction cells 14with the cell unit 1. The cells shared between the cell unit 1 and thecell unit 2 are depicted as “shared portion 2” in FIG. 7. The cell unit3 has the same structure as that of the cell unit 1. The cell unit 3 isadjacent to the cell unit 1 in the Y direction while sharing one pieceof the first exhaust gas introduction cell 12 and two pieces of thesecond exhaust gas introduction cells 12 with the cell unit 1. The cellsshared between the cell unit 1 and the cell unit 3 are depicted as“shared portion 1” in FIG. 7.

Meanwhile, FIG. 7 shows four segments H, I, J, and K that do not crossthe octagonal shape region of the exhaust gas emission cell 11, andhypothetical two segments L and M that cross the octagonal shape regionof the exhaust gas emission cell 11, among hypothetical segmentsconnecting the geometric centers of gravity of the respective octagonalshapes of the four pieces of the second exhaust gas introduction cells14. The “shared portion 2” is depicted by cross-hatching with segmentsin the same direction as that of the segment M, and the “shared portion1” is depicted by cross-hatching with segments in the same direction asthat of the segment L.

As shown in FIG. 7, an intersection of the two segments L and M isidentical with the geometric center of gravity of the exhaust gasemission cell 11.

With regard to the cell shapes in the honeycomb filter 20 shown in FIGS.1, 2A, 2B, and 6A to 6C, the exhaust gas emission cells 11 and thesecond exhaust gas introduction cells 14, except for the exhaust gasemission cells 11A and 11B adjacent to the outer wall 17, each have anoctagonal cross section, and the first exhaust gas introduction cells 12and 12A each have a square cross section. However, the cross sectionalshapes of the exhaust gas emission cells and the exhaust gasintroduction cells of the embodiment of the present invention are notlimited to the above shapes, and may be all square as mentioned below,or may be combinations of other polygons.

Moreover, the exhaust gas emission cells 11, the first exhaust gasintroduction cells 12, and the second exhaust gas introduction cells 14,which are polygons in the cross section thereof, may be roundly corneredso that the vertex portions are formed by curved lines in the crosssection.

Example of the curved lines include curved lines obtained by dividing acircle into quarters, and carved lines obtained by dividing an ellipseinto four equal parts linearly along the long axis and the axisperpendicular to the long axis. In particular, the vertex portions ofthe cells having a rectangular cross section are preferably formed bycurved lines in the cross section. This prevents stress concentration atthe corner portions, thereby preventing cracks in the cell walls.

Furthermore, the honeycomb filter 20 may partially include cells formedby curved lines such as a cell having a circle cross section.

The following description is not applicable to the exhaust gas emissioncells 11A and 11B adjacent to the cuter wall 17.

The cross sectional area of each first exhaust gas introduction cell 12is preferably 20 to 50%, and more preferably 22 to 45% the size of thecross sectional area of each second exhaust gas introduction cell 14.

In the honeycomb filter 20 shown in FIGS. 1, 2A, 2B, and 6A to 6C, thecross sectional area of each exhaust gas emission cell 11 is equal tothe cross sectional area of each second exhaust gas introduction cell14; however, the cross sectional area of each exhaust gas emission cell11 may be larger than the cross sectional area of each second exhaustgas introduction cell 14.

The cross sectional area of each exhaust gas emission cell 11 ispreferably 1.05 to 1.5 times the size of the cross sectional area ofeach second exhaust gas introduction cell 14.

Moreover, the side 13 a facing the exhaust gas emission, cell 11 amongthe sides forming the cross sectional shape of the first exhaust gasintroduction cell 12 is longer than the side 14 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell 14. The sides 12 a and 14 a aresides facing the exhaust gas emission cell 11 according to theaforementioned definition of the embodiments of the present invention.

The ratio of the length of the side 14 a of the second exhaust gasintroduction cell 14 to the length of the side 12 a of the first exhaustgas introduction cell 12 (length of the side 14 a/length of the side 12a) is not particularly limited, and is preferably not more than 0.8,more preferably not more than 0.7, and still more preferably not morethan 0.5.

As shown in FIG. 2B, exhaust gas G₁ (exhaust gas is represented by anarrow G₁ which shows the flow of exhaust gas in FIG. 2B) having flowedinto the first exhaust gas introduction cells 12 or the second exhaustgas introduction cells 14 inevitably passes through the cell walls 13which separates the exhaust gas emission cells 11 and the respectivefirst exhaust gas introduction cells 12 or the respective second exhaustgas introduction cells 14, and then flows out of the exhaust gasemission cells 11. Upon passage of exhaust gas G₁ through the cell walls13, PMs and the like in the exhaust gas are captured so that the cellwalls 13 function as filters.

The exhaust gas emission cells 11, the first exhaust gas introductioncells 12, and the second exhaust gas introduction cells 14 allow flow ofgas such as exhaust gas as described above. For flow of exhaust gas inthe direction shown in FIG. 2B, an end at a first end face 10 f of thehoneycomb fired body 10 (the end on the side at which the exhaust gasemission cells 11 are plugged) is referred to as an exhaust gasintroduction side end, an end at a second end face 10 r of the honeycombfired body 10 (the end on the side at which the first exhaust gasintroduction cells 12, and the second exhaust gas introduction cells 14are plugged) is referred to as an exhaust gas emission side end.

The honeycomb filter 20 having the aforementioned structure highlypurifies CO and HC and can reduce the initial pressure loss as comparedwith conventional honeycomb filters. Also, the honeycomb filter 20 canreduce the rate of increase in the pressure loss even after accumulationof a considerable amount of PMs on the cell walls as explained in theeffects of the honeycomb filter according to the embodiment of thepresent invention. The honeycomb filter 20 can significantly reduce thepressure loss throughout the use from the initial stage to afteraccumulation of PMs in close to the limit amount.

In the honeycomb filter 20 having the structure illustrated in FIGS. 2Aand 2B, since the thickness of the outer wall 17 is uniform ever thewhole length, the strength of the honeycomb fired body is enhanced bythe outer wall and the partial variation in the volume ratio is furtherreduced between the exhaust gas emission cells and the exhaust gasintroduction cells in the honeycomb fired body. Consequently, theuniform flow of exhaust gas is improved so that the pressure loss can bereduced.

In terms of the cells adjacent to the outer wall 17, since the exhaustgas emission cells 11A and 11B are each adjacent to the exhaust gasintroduction cell 12A, exhaust ear can pass through the inside of theouter wall 17, which means chat part of the outer wall 17 can be used asa filter. As a result, the pressure loss is further reduced.

Moreover, since the outer wall 17 is chamfered, stress is prevented fromconcentrating on the corner portions of the honeycomb fired body 10, sothat cracks hardly occur in the corner portions of the honeycomb firedbody 10. When the cross sectional shape or the exhaust gas emission cell11B positioned at the corner portion is a pentagon formed by straightlines, not having the chambered portion 110B formed by curved lines, theexhaust gas emission cell 11B positioned close to the corner portion ofthe honeycomb fired body 10 is likely to have a stress concentratedthereon to easily have cracks. In the honeycomb filter 20, however,since the exhaust gas emission cells 11B each have the chamfered portion110B, cracks hardly occurs therein.

The honeycomb filter 20 according to the first embodiment includes aplurality of honeycomb fired bodies 10. Examples of the material for thehoneycomb fired bodies 10 include carbide ceramics such as siliconcarbide, titanium carbide, tantalum carbide, and tungsten carbide;nitride ceramics such as aluminum nitride, silicon nitride, boronnitride, and titanium nitride; oxide ceramics such as alumina, zirconia,cordierite, mullite, and aluminum titanate; and silicon-containingsilicon, carbide, and the like. Silicon carbide or silicon-containingsilicon carbide is preferable among the examples as they are excellentin heat resistance, mechanical strength, thermal conductivity and thelike.

Meanwhile, the silicon-containing silicon carbide is a mixture ofsilicon carbide and silicon metal, and is preferably silicon-containingsilicon carbide including 60 wt % or more of silicon carbide.

The thickness of the cell walls separating the cells is preferablyuniform at any part in the honeycomb fired body 10 forming the honeycombfilter 20 according to the first embodiment. The thickness of the cellwalls is preferably 0.10 to 0.46 mm, and more preferably 0.12 to 0.35mm. The thickness of the outer wall 17 is preferably 0.10 to 0.50 mm.Meanwhile, the thickness of the cell wall is a value measured as thethickness D shown in FIG. 3B based on the aforementioned definition.

The thickness of the plugged portion is preferably 1.0 to 5.0 mm.

The average pore diameter of pores in the cell walls is preferably 8 to25 μm in the honeycomb fired body 10 forming the honeycomb filter 20according to the first embodiment.

The number of the cells per unit area is preferably 31 to 62 pcs/cm²(200 to 400 pcs/inch²) in the cross section of the honeycomb fired body10.

The honeycomb filter 20 according to the embodiment of the presentinvention is formed by combining, with an adhesive layer residingtherebetween, a plurality of the honeycomb fired bodies each having anouter wall on the periphery thereof. An adhesive layer that combines thehoneycomb fired bodies is prepared by applying an adhesive paste thatcontains an inorganic binder and inorganic particles, and drying theadhesive paste. The adhesive layer may further contain at least one ofan inorganic fiber and a whisker.

The adhesive layer preferably has a thickness of 0.5 to 2.0 mm.

The honeycomb filter according to the first embodiment of the presentinvention, may have a periphery coat layer on the periphery thereof. Thematerial of the periphery coat layer is preferably the same as thematerials of the adhesive layer.

The periphery coat layer preferably has a thickness of 0.1 to 3.0 mm.

The following will discuss a method of manufacturing the honeycombfilter according to the first embodiment of the present invention.

The method described below uses a silicon carbide as ceramic powder.

(1) A molding process for manufacturing a honeycomb molded body isperformed by extrusion molding a wet mixture containing ceramic powderand a binder.

Specifically, silicon carbide powders having different average particlesizes serving as ceramic powder, an organic binder, a pore-formingagent, a liquid plasticizer, a lubricant, and water are mixed to preparea wet mixture for manufacturing a honeycomb molded body.

Examples of the pore-forming agent include balloons that are fine hollowspheres including oxide-based ceramics, spherical acrylic particles,graphite, and starch.

The balloons are not particularly limited, and examples thereof includealumina balloon, glass micro balloon, shirasu balloon, fly ash balloon(FA balloon), and mullite balloon. Alumina balloon is preferable amongthese.

The average particle diameter of the pore-forming agent is preferably 30to 50 μm, more preferably 35 to 45 μm, and still more preferably 1.5 to2.5 times the target average pore diameter of the honeycomb filter. Thepore-forming agent preferably includes a pore-forming particle having aparticle diameter of not more than 20 μm and a pore-forming particlehaving a particle diameter of not less than 60 μm, each in an amount ofnot more than 10% and preferably each in an amount of not more than 5%,relative to all the pore-forming particles. The pore-forming agentcontent is preferably 10 to 25 wt % and more preferably 15 to 20 wt %relative to the silicon carbide powders serving as ceramic powder.

Then, the wet mixture is charged into an extrusion molding machine andextrusion-molded to manufacture honeycomb molded bodies in predeterminedshapes.

Here, a honeycomb molded body is manufactured with a die that can make across sectional shape having the cell structure (shapes and arrangementof the cells) shown in FIGS. 2A and 2B.

(2) The honeycomb molded bodies are cut at a predetermined length anddried with use of a drying apparatus such as a microwave dryingapparatus, a hot-air drying apparatus, a dielectric drying apparatus, areduced-pressure drying apparatus, a vacuum drying apparatus, or afreeze crying apparatus. Then, predetermined cells are plugged byfilling the cells with a plug material paste to be a plug (pluggingprocess).

Here, the wet mixture may be used as the plug material paste.

(3) Then, the honeycomb molded body is heated at 300° C. to 650° C. in adecreasing furnace to remove organic matters in the honeycomb moldedbody (degreasing process). The decreased honeycomb molded body istransferred to a firing furnace and fired at 2000° C. to 2200° C.(firing process). In this manner, the honeycomb fired body having theconfiguration as shown in FIGS. 2A and 2B is manufactured.

The plug material paste filled into the end of the cells is fired byheating to be a plug.

Conditions for cutting, drying, plugging, degreasing, and firing may beconditions conventionally used for manufacturing honeycomb fired bodies.

(4) A plurality of the honeycomb fired bodies are stacked in series withthe adhesive paste residing therebetween on a support table to combinethe honeycomb fired bodies (combining process) so that a honeycombaggregate body including the plurality of stacked honeycomb fired bodiesis manufactured.

The adhesive paste contains, for example, an inorganic binder, anorganic binder, and inorganic particles. The adhesive paste may furthercontain inorganic fibers and/or whisker.

Examples of the inorganic particles in the adhesive paste includecarbide particles, nitride particles, and the like. Specific examplesthereof include inorganic particles made from silicon carbide, siliconnitride, boron nitride, and the like. Each of these may be used alone,or two or more of these may be used in combination. Among the inorganicparticles, silicon carbide particles are preferable due to theirsuperior thermal conductivity.

Examples of the inorganic fibers and/or whisker in the adhesive pasteinclude inorganic fibers and/or whisker made from silica-alumina,mullite, alumina, and silica. Each of these may be used alone or two ormore of these may be used in combination. Alumina fibers are preferable,among the inorganic fibers. The inorganic fibers may be biosolublefibers.

Furthermore, balloons that are fine hollow spheres including oxide-basedceramics, spherical acrylic particles, graphite, or the like may beadded to the adhesive paste, if necessary. The balloons are notparticularly limited, and examples thereof include alumina balloon,glass micro balloon, shirasu balloon, fly ash balloon (FA balloon),mullite balloon, and the like.

(5) The honeycomb aggregate body is then heated to solidify the adhesivepaste, whereby a rectangular pillar-shaped ceramic block ismanufactured.

The heating and solidifying of the adhesive paste may be performed underconditions that have been conventionally employed for manufacturinghoneycomb filters.

(6) The ceramic block is subjected to cutting (cutting process).

Specifically, the periphery of the ceramic block is cut with a diamondcutter, whereby a ceramic block whose periphery is cut into asubstantially round pillar shape is manufactured.

(7) A peripheral coating material paste is applied to the peripheralface of the substantially round pillar-shaped ceramic block, and isdried and solidified to form a periphery coat layer (periphery coatlayer forming process).

The adhesive paste may be used as the peripheral coating material paste.Alternatively, the peripheral coating material paste may be a pastehaving a composition different from that of the adhesive paste.

The periphery coat layer is not necessarily formed and may be formed, ifnecessary.

The peripheral shape of the ceramic block is adjusted by the peripherycoat layer, and thereby a round pillar-shaped honeycomb filter isobtained.

The honeycomb filter including the honeycomb fired bodies can bemanufactured through the aforementioned processes.

Although the honeycomb filter having a predetermined shape ismanufactured by cutting, the honeycomb filter may also be allowed tohave a predetermined shape such as round-pillar shape as follows:honeycomb fired bodies of a plurality of shapes, each having an octetwall on the periphery thereof, are manufactured in the honeycomb firedbody manufacturing process, and then the honeycomb fired bodies of aplurality of shapes are combined with one another with the adhesivelayer residing therebetween. In this case, the cutting process can beomitted.

(8) The honeycomb filter is allowed to support an oxidation catalyst(catalyst coating process).

Gamma alumina that supports platinum described above may be used as anoxidation catalyst. The oxidation catalyst is mixed with a solvent toform slurry. The honeycomb filter is immersed in the slurry so that theoxidation catalyst is supported in the honeycomb filter. The amount ofthe oxidation catalyst supported can be controlled by repetition of aseries of immersion in the slurry, air blow, and drying.

The oxidation catalyst can be supported inside the cell walls by thefollowing method: gamma alumina which already supports platinum and iscontrolled to have an average secondary particle diameter of 0.5 to 3 μmis made into slurry; and a honeycomb fired body is immersed in theslurry. The gamma alumina which supports platinum can be prepared byimmersing gamma aluminum particles in a solution of a metal compoundcontaining platinum. The amount of the oxidation catalyst to besupported on the surface of the cell walls can be reduced by controllingthe viscosity of the slurry and controlling the force of the air blow orsuction.

Hereinafter, the effects of the honeycomb filter according to the firstembodiment of the present invention are listed.

(1) In the honeycomb filter according to the present embodiment, anoxidation catalyst is supported inside the cell walls in an amount of 5to 60 g/L; the exhaust gas emission cells have a larger average crosssectional area perpendicular to the longitudinal direction than theexhaust gas introduction cells in a direction perpendicular to thelongitudinal direction; and the total volume of the exhaust gasintroduction cells is greater than the total volume of the exhaust gasemission cells. Thus, flow-in exhaust gas easily contacts with theoxidation catalyst supported on the cell walls, thereby achieving a highgas purification performance.

(2) The honeycomb filter according to the embodiment can not only reducethe initial pressure loss as compared with conventional honeycombfilters but also reduce the rate of increase in the pressure loss evenafter accumulation of a considerable amount of PMs on the cell walls.The honeycomb filter can significantly reduce the pressure lossthroughout the use from the initial stage to after accumulation of PMsin close to the limit amount.

(3) In the honeycomb filter according to the present embodiment, thecross sectional area of each first exhaust gas introduction cell may be20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

This setting of the cross sectional area ratio of the first exhaust gasintroduction cell and the second exhaust gas introduction cell canprovide difference between the resistance caused upon flowing of exhaustgas through the first exhaust gas introduction cells and the resistancecaused upon flowing of exhaust gas through the second exhaust gasintroduction cells, thereby enabling effective reduction of the pressureloss.

(4) In the honeycomb filter according to the present embodiment, theratio of the length of the side of the second exhaust gas introductioncell facing the exhaust gas emission cell to the length of the side ofthe first exhaust gas introduction cell facing the exhaust gas emissioncell may be not more than 0.8.

The aforementioned ratio of the length of the side of the first exhaustgas introduction cell to the length of the side of the second exhaustgas introduction cell enables exhaust gas to easily pass through thecell walls separating the exhaust gas emission cells and the firstexhaust gas introduction cells, effective suppression of the initialpressure loss, and prevention of an increase in the rate of increase orthe pressure loss after accumulation of PMs.

(5) In the honeycomb filter according to the present embodiment, thematerial of the honeycomb fared bodies may include silicon carbide orsilicon-containing silicon carbide. Such a material enables to provide ahoneycomb filter having excellent heat resistance.

(6) In the honeycomb filter according to the present embodiment, thethickness of the cell walls separating the cells may be uniform at anypart.

This setting for the entire thickness of the cell walls enables toprovide a honeycomb filter having the same effects at any part thereof.

(7) In the honeycomb filter according to the present embodiment, thethickness of the cell walls may be 0.10 to 0.46 mm.

The cell walls having the aforementioned thickness are sufficient forcapturing PMs in exhaust gas, and also enable efficient suppression ofincrease in the pressure loss.

(8) In the honeycomb filter according to the present embodiment, theaverage pore diameter of pores in the cell walls may be 8 to 25 μm inthe honeycomb fired body forming the honeycomb filter.

The above average pore diameter of pores in the cell walls enables tocapture PMs at a high capturing efficiency while suppressing an increasein the pressure loss.

(9) In the honeycomb filter according to the present embodiment, theexhaust gas introduction cells and the exhaust gas emission cells eachhave a uniform cross sectional shape except for the plugged portion in adirection perpendicular to the longitudinal direction of the cellsthoroughly from the end at the exhaust gas introduction side to the endat the exhaust gas emission side.

Thus, the entire honeycomb filter can exert the same or similar effectsso that disadvantages caused by local shape variations in the honeycombfilter can be prevented.

(10) In the honeycomb flitter according to the present embodiment,cracks tend not to be caused by thermal shock that occurs uponburn-removal (regeneration) of accumulated PMs in the honeycomb filter.

In the honeycomb filter according to the present embodiment, the pluggedportions provided at the end at the exhaust gas emission side, namelythe plugged portions in the first exhaust gas introduction cells and thesecond exhaust gas introduction cells are present in vertical andhorizontal rows. Each plugged portion has a width equal to or largerthan the length of one side of the first exhaust gas introduction cell.In ordinary regeneration of honeycomb filters, heat burns firstly PMsaccumulated at the exhaust gas introduction side of a honeycomb filter.Then, burning of PMs is transferred by exhaust gas flow to the emissionside of the honeycomb filter so that all the PMs are burned. For thisreason, positions closer to the emission side of the honeycomb filterare exposed to higher temperatures, and easily have temperaturedifference along the diameter direction of the honeycomb filter. Thus,cracks occur due to the thermal stress. Such cracks are prominent in thecase where the cross sectional areas are different among the cells, andalso in the case where the cross sections of the exhaust gasintroduction cells where PMs accumulate have square cross section(s). Incontrast, in the honeycomb filter according to the embodiment of thepresent invention, the plugged portions aligned in vertical andhorizontal rows at the exhaust gas emission side function as thermallyconductive layers and as layers for dissipating heat to outside. Thus,the temperature difference is small along the diameter direction at theand of the exhaust gas emission side of the honeycomb filter. Thisreduces the thermal stress to prevent occurrence of cracks. Moreover, inthe honeycomb filter according to the embodiment of the presentinvention, the amount of the plug in the second exhaust gas introductioncells, in which a larger amount of PMs accumulate compared with thefirst exhaust gas introduction cells, is large due to the size of thecross sectional area, and thereby the thermal capacity of the pluggedportions increase. Such plugged portions can prevent an increase in thetemperature of the second exhaust gas introduction cells, where the heatof burning PMs is higher, even in consideration of each cell. Thisreduces the temperature difference in the diameter direction of thehoneycomb filter, thereby presumably reducing the thermal stress to becaused.

Thus, the honeycomb filter according to the present embodiment canprevent cracks during the regeneration even though it includes exhaustgas introduction cells having square cross sections.

(11) In the honeycomb filter according to the embodiment, theaforementioned structure enables to not only enhance the strength of thehoneycomb fired body by the outer wall but also further reduce thepartial variation in the volume ratio between the exhaust gas emissioncells and the exhaust gas introduction cells in the honeycomb firedbody. Consequently, the uniform flow of exhaust gas is improved so thatthe pressure loss can be reduced.

Hereinafter, examples are given for more specifically describing thefirst embodiment of the present invention. However, the presentinvention is not limited only to the examples.

Example 1

A mixture was obtained by mixing 56.3% by weight of a silicon carbidecoarse powder having an average particle size of 22 μm and 24.1% byweight of a silicon carbide fine powder having an average particle sizeof 0.5 μm. To the mixture were added 4.4% by weight of an organic binder(methylcellulose), 0.8% by weight of a lubricant (UNILUB, manufacturedby NOF Corporation), 0.8% by weight of glycerin, 2.2% by weight of oleicacid, and 11.3% by weight of water and then kneaded to prepare a wetmixture. Thereafter, the wet mixture was extrusion-molded (moldingprocess).

This process provided a raw honeycomb molded body which had the sameshape as that of the honeycomb fired body 10 shown in FIG. 2A and inwhich the cells were not plugged.

Next, the raw honeycomb molded body was dried using a microwave dryingapparatus to obtain dried honeycomb molded bodies. Then, predeterminedcells of the dried honeycomb molded body were plugged by filling thecells with a plug material paste.

Specifically, the cells are plugged in a manner that the end at theexhaust gas introduction side and the end at the exhaust gas emissionside are plugged at the positions shown in FIG. 6A.

The wet mixture was used as the plug material paste. Thereafter, thedried honeycomb molded body, which has predetermined cells filled withthe ping material paste, was dried with a drying apparatus again.

Subsequently, the dried honeycomb molded bodies after plugging of cellswere decreased at 400° C. (decrease treatment) and then fired at 2200°C. (firing treatment) under normal pressure argon atmosphere for threehours.

In this manner, a rectangular pillar-shaped honeycomb fired body wasmanufactured.

The measurement mentioned below of the length of sides and the crosssectional area can be performed by the aforementioned image analysis ofan electron microscope photograph and by use of the aforementioned grainsite distribution measurement software (Mac-View (version 3.5), producedby Mountech Co. Ltd.).

The manufactured honeycomb fired body was the honeycomb fired body 10shown in FIGS. 2A and 2B formed of a silicon carbide sintered bodyhaving a porosity of 42%, an average pore diameter of 11 μm, a size of34.3 mm×34.3 mm×177.8 mm, the number of cells (cell density) of 310pcs/inch², a thickness of cell walls of 0.18 mm, and a thickness of eachplugged portion of 3 mm.

The exhaust gas emission cell 11 was adjacently surrounded fully by thefirst exhaust gas introduction cells 12 and 12A and the second exhaustgas introduction cells 14 in the cross section perpendicular to thelongitudinal direction of the manufactured honeycomb fired body 10. Thefirst exhaust gas introduction cells 12 and 12A had square crosssections, and the length of sides forming the cross sections of thefirst exhaust gas introduction cells 12 and 12A was 1.02 mm.

The second exhaust gas introduction cells 14 had octagonal crosssections. The length of the hypotenuse side of the second exhaust gasintroduction cells facing the exhaust gas emission cell 11 was 0.32 mm,and the vertical or horizontal sides not facing the exhaust gas emissioncell 11 were 1.13 mm.

In other words, the length of the side facing the exhaust gas emissioncell 11 among the sides forming the cross sectional shape of the secondexhaust gas introduction cell 14 was 0.28 times as long as than the sidefacing the exhaust gas emission cell 11 among the sides forming thefirst exhaust gas introduction cell 12.

In the exhaust gas emission cells 11B at the four corners, the length ofsides adjacent to the outer wall 17 was 1.30 mm, the length of thevertical or horizontal sides was 1.08 mm, the length of the hypotenusesides was 0.32 mm, and the cross sectional area was 1.67 mm².

In the exhaust gas emission cells 11A, the length of the side adjacentto the outer wall 17 was 1.58 mm, the length of the vertical sideparallel to the side adjacent to the outer wall 17 was 1.13 mm, thelength of the horizontal side connected at a right angle to the sideadjacent to the outer wall 17 was 1.08 mm, the length of the hypotenuseside was 0.32 mm, and the cross sectional area was 2.00 mm².

The exhaust gas emission cell 11 had an octagonal cross section, and theshape of the cross section was the same as that of the second exhaustgas introduction cell 14. The length of the hypotenuse side facing thesecond exhaust gas introduction cell 14 was 0.32 mm, and the vertical orhorizontal sides facing the first exhaust gas introduction cells 12 was1.13 mm.

The thickness of the outer wall 17 was 0.35 mm.

The cross sectional area of the first exhaust gas introduction cell 12was 1.05 mm², and the cross sectional areas of the second exhaust gasintroduction cell 14 and the exhaust gas emission cell 11 were both 2.39mm². In other words, the cross sectional area of the first exhaust gasintroduction cell 12 was 44% the sere of the cross sectional area of thesecond exhaust gas introduction cell 14.

Moreover, the cross sectional area of the exhaust gas emission cell 11was equal in size to the cross sectional area of the second exhaust gasintroduction cell 14, and was larger than the cross sectional area ofthe first exhaust gas introduction cell 12.

The honeycomb fired body was in a rectangular pillar shape that wasroundly cornered at the corner portions in end faces thereof.

Next, a plurality of the honeycomb fired bodies were combined using anadhesive paste containing 30% by weight of alumina fibers having anaverage fiber length of 20 μm, 21% by weight of silicon carbideparticles having an average particle size of 0.6 μm, 15% by weight ofsilica sol, 5.6% by weight of carboxymethyl cellulose, and 28.4% byweight of water. Subsequently, the adhesive layer was dried andsolidified at 120° C. to form an adhesive layer, and thereby apillar-shaped ceramic block was manufactured.

The periphery of the pillar-shaped ceramic block was cut out using adiamond cutter to manufacture a substantially round pillar-shapedceramic block.

Subsequently, a sealing material paste having the same composition asthat of the adhesive paste was applied to the peripheral face of theceramic block. The sealing material paste was dried and solidified at120° C. to form a periphery coat layer. In this manner a roundpillar-shaped honeycomb filter was manufactured.

The diameter of the honeycomb filter was 266.7 mm, and the length in thelongitudinal direction was 177.8 mm.

Subsequently, the honeycomb filter is immersed in slurry containingdispersed gamma alumina which has an average particle diameter of 2 μmand supports platinum, as an oxidation catalyst. Then, the honeycombfilter is dried at 120° C. and heated at 450° C., and thereby thehoneycomb filter supports 219 g (22 g/L in total of 20 g/L of gammaalumina and 2 g/L of platinum) of the oxidation catalyst inside the cellwalls.

Comparative Example 1

A raw honeycomb molded body was obtained in the same molding process asin Example 1. Subsequently the raw honeycomb molded body was dried usinga microwave drying apparatus to manufacture a dried honeycomb moldedbody. Then, predetermined cells of the dried honeycomb molded body wereplugged by filling the cells with a plug material paste.

The positions of the cells to be plugged were changed from those inExample 1 as follows. The octagonal cells were all plugged in the endface corresponding to the end at the exhaust gas emission side, and thesquare cells were all plugged in the end face corresponding to the endat the exhaust gas introduction side so that the cells were alternatelyplugged in both of the end faces.

Consequently, a honeycomb molded body was obtained in which the end atthe exhaust gas introduction side and the end at the exhaust gasemission side were plugged at the positions shown in FIG. 21B.

Honeycomb fired bodies 130 and a honeycomb filter 120 shown in FIGS. 21Aand 21B were manufactured through the same processes as in Example 1.The same oxidation catalyst in the same amount as in Example 1 wassupported in the honeycomb filter 120.

In the cross section of the manufactured honeycomb fired body 130 in thedirection perpendicular to the longitudinal direction of the cells, allthe exhaust gas introduction cells 132 had an octagonal cross sectionexcept for the exhaust gas introduction cells 132A and 132B which wereadjacent to the outer wall 137.

The sides facing the exhaust gas emission cell 131 were vertical orhorizontal sides having a length of 1.13 mm.

The sides facing other exhaust gas introduction cells 132, 132A, and132B were hypotenuse sides having a length of 0.32 mm.

All the exhaust gas emission cells 131 and 131A had a square crosssection. The length of the sides forming the cross sections of theexhaust gas emission cells 131 and 131A was 1.02 mm.

In the exhaust gas introduction cells 132B at the four corners, thelength of sides adjacent to the outer wall 137 was 1.30 mm, the lengthof the vertical or horizontal sides was 1.08 mm, the length of thehypotenuse sides was 0.32 mm, and the cross sectional area was 1.67 mm².

In the exhaust gas introduction cells 132A, the length of the sideadjacent to the outer wall 137 was 1.58 mm, the length of the verticalside parallel to the side adjacent to the outer wall 17 was 1.13 mm, thelength of the horizontal line connected at a right angle to the sideadjacent to the outer wall 17 was 1.08 mm, the length of the hypotenuseside was 0.32 mm, and the cross sectional area was 2.00 mm².

The thickness of the cell walls 133 was 0.18 mm, and the thickness ofthe outer walls was 0.35 mm.

The cross sectional area of the exhaust gas introduction cell 132 was2.39 mm², and the cross sectional, area of the exhaust gas emission cell131 was 1.05 mm². In other words, the cross sectional area of theexhaust gas introduction cell 132 was larger than the cross sectionalarea of the exhaust gas emission cell 131.

(Measurement of Initial Pressure Loss)

FIG. 8 is an explanatory diagram schematically illustrating a method formeasuring the initial pressure loss.

An initial pressure loss measuring apparatus 210 includes a blower 211,an exhaust gas pipe 212 connected to the blower 211, a metal casing 213in which the honeycomb filter 20 is fixed, and a pressure gauge 214 inwhich pipes are arranged so that the pressure in front and back of thehoneycomb filter 20 can be measured. Specifically, in the initialpressure loss measuring apparatus 210, the pressure loss is measured byflowing gas through the honeycomb filter 20 and measuring the pressurein front and back of the honeycomb filter.

The blower 211 is operated three times to flow gas at a rate of 600m³/h, 800 m³/h, and 1200 m³/h. In each operation, the pressure loss infive minutes from the start is measured.

FIG. 10B is a graph showing the relations between the gas flow rates andthe initial pressure losses of the honeycomb filters manufactured inExample 1 and Comparative Example 1.

The graph in FIG. 10B clearly shows that, in the honeycomb filter ofComparative Example 1, the initial pressure loss was 0.73 Pa, 1.13 Pa,and 2.27 Pa for the gas flow rate of 600 m³/h, 800 m³/h, and 1200 m³/h,respectively. In the honeycomb filter of Example 1, the initial pressureloss was 0.60 Pa, 0.91 Pa, and 1.77 Pa for the gas flow rate of 600m³/h, 800 m³/h, and 1200 m³/h, respectively, which was lower than thatin Comparative Example 1. In particular, as the flow rate increases, thedifference from Comparative Example 1 becomes more significant.

(Measurement of Pressure Loss)

FIG. 9 is an explanatory diagram schematically illustrating a method formeasuring the pressure loss.

The pressure loss measuring apparatus 310 has the following structure: ahoneycomb filter 20 fixed inside a metal casing 313 is disposed in anexhaust gas tube 312 of a 12.8-liter diesel engine 311, and a pressuregauge 314 is attached in a manner such that it can detect the pressurein front and back of the honeycomb filter 20. The honeycomb filter 20 isdisposed such that the end at the exhaust gas introduction side iscloser to the exhaust gas tube 312 of the diesel engine 311. Namely, thehoneycomb filter 20 is disposed to allow exhaust gas to flow in thecells which are open at the exhaust gas introduction side end.

The diesel engine 311 is operated with the number of rotation of 1800rpm and a torque of 2000 Nm to allow exhaust gas to flow into thehoneycomb filter 20 so that PMs are captured by the honeycomb filter.

Then, a relation is determined between the amount (g/L) of captured PMsper liter of an apparent volume of the honeycomb filter and the pressureloss (kPa).

FIG. 10A is a graph showing the relations between the PM capture amountsand the pressure losses of the honeycomb filters manufactured in Example1 and Comparative Example 1.

The graph in FIG. 10A clearly shows that the initial pressure loss, i.e.the pressure loss when the PM capture amount is 0 g/L, is as low as 2.7kPa, and that the pressure loss is as low as 6.6 kPa even when the PMcapture amount is 6 g/L in the honeycomb filter of Example 1. Thus, thehoneycomb filter of Example 1 has a significant effect of achieving alower pressure loss, in addition to a lower initial pressure loss, ascompared to the honeycomb filter of Comparative Example 1 at any timewhen the PM capture amount was 0 g/L to 6 g/L. In the honeycomb filterof Comparative Example 1, the initial pressure loss, i.e. the pressureloss when the PM capture amount was 0 g/L, is 3.6 kPa, and the pressureloss is 7.6 kPa when the PM capture amount was 6 g/L.

Second Embodiment

The following will discuss a honeycomb filter according to the secondembodiment of the present invention.

The honeycomb filter according to the second embodiment of the presentinvention includes a single honeycomb fired body having an outer wall onthe periphery thereof. The honeycomb fired body includes exhaust gasemission cells each having an open end at an exhaust gas emission sideand a plugged end at an exhaust gas introduction side, and exhaust gasintroduction cells each having an open end at the exhaust gasintroduction side and a plugged end at the exhaust gas emission side,the exhaust gas introduction cells including first exhaust gasintroduction cells and second exhaust gas introduction cells. In thehoneycomb filter, an oxidation catalyst is supported inside the cellwalls in an amount of 5 to 60 g/L.

The exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction cell, and eachexhaust gas emission cell has the same cross sectional area as eachsecond exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells and the exhaust gas introductioncells are each polygonal, and a side facing one exhaust gas emissioncell among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is longer than a side facing one exhaustgas emission cell among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except for the pluggedportion, in a direction perpendicular to the longitudinal direction ofthe cells thoroughly from the end at the exhaust gas introduction sideto the end at the exhaust gas emission side.

Namely, the honeycomb filter according to the second embodiment has thesame structure as that of the honeycomb filter according to the firstembodiment except that the honeycomb filter according to the secondembodiment is formed of a single honeycomb fired body having an outerwall on the periphery thereof. Such a honeycomb filter formed of asingle honeycomb fired body is referred to also as an integratedhoneycomb filter.

FIG. 11A is a perspective view schematically illustrating one example ofthe integrated honeycomb filter according to the second embodiment ofthe present invention. FIG. 11B is a B-B line cross sectional view ofthe integrated honeycomb filter.

In the honeycomb filter 30 shown in FIG. 11A and FIG. 11B, exhaust gasemission cells 31 having an octagonal cross section are each adjacentlysurrounded fully by first exhaust gas introduction cells 32 each havinga square cross section and second exhaust gas introduction cells 33 eachhaving an octagonal cross section across porous cell walls therebetween.The first exhaust gas introduction cells 32 and the second exhaust gasintroduction cells 34 are alternately arranged around each exhaust gasemission cell 31. Each second exhaust gas introduction cell 34 has alarger cross section than each first exhaust gas introduction cell 32,and each exhaust gas emission cell 31 has the same cross sectional areaas each second exhaust gas introduction cell 34.

The length of a side 32 a facing the exhaust gas emission cell 31 amongthe sides forming the cross sectional shape of the first exhaust gasintroduction cell 32 is longer than a side 34 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell 34.

The honeycomb filter 30 according to the present embodiment is formed ofa single honeycomb fired body, and has an outer wall 37 on the peripheryof the honeycomb fired body. Preferable examples of the material formingthe honeycomb filter 30 according to the present embodiment includecordierite and aluminum titanate. These materials have a low coefficientof thermal expansion. Thus, cracks or the like caused by thermal stressduring regeneration or the like rarely occur even in a large scalehoneycomb filter.

The features of the embodiment other than those mentioned above are thesame as the features described in relation to the first embodiment, andthus the explanation thereof is omitted.

The honeycomb filter 30 according to the present embodiment can bemanufactured by using the honeycomb fired body manufactured in the firstembodiment as it is, or can be manufactured in the same manner as in thefirst embodiment of the present invention, except that a periphery coatlayer is formed on the periphery thereof. This embodiment does notrequire the processes (4), (5), and (6) in the method for manufacturingthe honeycomb filter according to the first embodiment of the presentinvention. Furthermore, the process (7) is not necessary in the case ofnot forming the periphery coat layer.

The honeycomb filter 30 according to the present embodiment issubstantially the same as the honeycomb filter 20 according to the firstembodiment concerning the manners of basic arrangement of cells, shapes,clogging, and the like. Thus, the honeycomb filter 30 according to thepresent embodiment can exert the same effects as the effects (1) to (10)described, in relation to the first embodiment.

Third Embodiment

The following will discuss a honeycomb filter according to the thirdembodiment of the present invention.

The honeycomb filter according to the third embodiment of the presentinvention includes a plurality of honeycomb fired bodies having an outerwall on the periphery thereof, the honeycomb fired bodies combined withone another by adhesive layers residing therebetween. Each honeycombfired body includes exhaust gas emission cells each having an open endat an exhaust gas emission side and a plugged end at an exhaust gasintroduction side, and exhaust gas introduction cells each having anopen end at the exhaust gas introduction side and a plugged end at theexhaust gas emission side, the exhaust gas introduction cells includingfirst exhaust gas introduction cells and second exhaust gar introductioncells. In the honeycomb filter, an oxidation catalyst is supportedinside the cell walls in an amount of 0 to 60 g/L.

The exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction cell, and eachexhaust gas emission cell has the same cross sectional area as eachsecond exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells and the exhaust gas introductioncells are each polygonal, and a side facing one exhaust gas emissioncell among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is longer than a side facing one exhaustgas emission cell among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell.

In relation to the cells adjacent to the outer wall, the exhaust gasemission cells and the first exhaust gas introduction cells arealternately arranged.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except for the pluggedportions, in a direction perpendicular to the longitudinal direction ofthe cells thoroughly from the end at the exhaust gas introduction sideto the end at the exhaust gas emission side.

Namely, the honeycomb filter according to the third embodiment has thesame structure as that of the honeycomb filter according to the firstembodiment. They are the same in terms of the basic shape endarrangement of the cells but different in that, in the honeycomb filteraccording to the third embodiment, the cross sectional shape of thecells adjacent to the outer wall is the same as the cross sectionalshape of the cells other than the cells adjacent to the outer wall.

FIG. 12A is a perspective view schematically illustrating one example ofa honeycomb filter according to a third embodiment of the presentinvention. FIG. 12B is a perspective view illustrating a honeycomb firedbody forming the honeycomb filter.

The cross sectional shape and arrangement of the cell walls in ahoneycomb fired body 10 a included in a honeycomb filter 30 shown inFIGS. 12A and 12B are basically the same as those of the cell walls inthe honeycomb fired body 10 included in the honeycomb filter 20 shown inFIG. 1. Exhaust gas emission cells 11 having an octagonal crosssectional shape are each adjacently surrounded fully by first exhaustgas introduction cells 12 having a square cross sectional shape andsecond exhaust gas introduction cells 14 having an octagonal crosssectional shape across porous cell walls. The first exhaust gasintroduction cells 12 and the second exhaust gas introduction cells 14are alternately arranged around the exhaust gas emission cells 11. Eachsecond exhaust gas introduction cell 14 has a larger cross sectionalarea than each first exhaust gas introduction cell 12. Each exhaust gasemission cell 11 has the same cross sectional area as each secondexhaust gas introduction cell 14. The honeycomb fired body 10 has anouter wall 17 a on the periphery thereof. The cells adjacent to theouter wall 17 a include the first exhaust gas introduction cells 12 andthe exhaust gas emission cells 11.

A side 12 a facing one exhaust gas emission cell 11 among the sidesforming the cross sectional shape of the first exhaust gas introductioncell 12 is longer than a side 14 a facing one exhaust gas emission cell11 among the sides forming the cross sectional shape of the secondexhaust gas introduction cell 14.

The honeycomb fired body 10 a is different from the honeycomb fired body10 included in the honeycomb filter 20 shown in FIG. 1 in that, as shownin FIG. 12B, the exhaust gas emission cells 11 and the first exhaust gasintroduction cells 12 adjacent to the outer wall respectively have thesame cross sectional shapes as the exhaust gas emission cells 11 and thefirst exhaust gas introduction cells 12 other than the cells adjacent tothe outer wall.

The following will disease a modified example of the honeycomb filteraccording to the third embodiment of the present invention.

FIG. 13A is a perspective view schematically illustrating one modifiedexample of a honeycomb fired body forming the honeycomb filter accordingto the third embodiment of the present invention. FIG. 13B is an endface view or a honeycomb fired body forming the honeycomb filter in FIG.13A.

In a honeycomb fired body 10 b in FIGS. 13A and 13B forming thehoneycomb filter shown, all the first exhaust gas introduction cells 12have the same shape and all the second exhaust gas introduction cells 14have the same shape. A side 170 b corresponding to an outer periphery ofan outer wall 17 b changes in accordance with the shapes of the firstexhaust gas introduction cells 12 and the exhaust gas emission cells 11adjacent to the outer wall 17 b. The thickness of the outer wall 17 b isuniform.

In other words, to make the thickness of the outer wall 17 b uniform,the side 170 b corresponding to the outer periphery of the outer wall 17b becomes rugged along with the shapes of the first exhaust gasintroduction cells 12 and the exhaust gas emission cells 11 adjacent tothe outer wall 17 b.

FIG. 13C is an end face view illustrating another modified example of ahoneycomb fired body forming the honeycomb filter according to the thirdembodiment of the present invention.

In a honeycomb fired body 10 c shown in FIG. 13C, all the cells adjacentto an outer wall 17 c are exhaust gas introduction cells. The shape of asecond exhaust gas introduction cell 14A is, compared to the shape of anexhaust gas introduction cell not adjacent to the outer wall 17 c,partially deformed to be hexagonal in accordance with the line along theinner wall forming the outer wall 17 c of first exhaust gas introductioncells 12A adjacent to the outer wall 17 c. Similarly, an exhaust gasemission cell 14B at the corner portion is partially deformed to bepentagonal.

As above, all the cells adjacent to the outer wall may be exhaust gasintroduction cells with an aim of enhancing the aperture ratio of theexhaust gas introduction cells.

The features of the embodiment other than those mentioned above are thesame as the features described in relation to the first embodiment, andthus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can bemanufactured by the same method as described in the first embodiment ofthe present invention, except that the shape of the mold used in theextrusion molding is changed.

Being similar to the honeycomb filter 20 of the first embodiment in thebasic arrangement and shapes of the cells and the state of plugs, thehoneycomb filter according to the present embodiment can exert the sameeffects as the effects (1) to (11) mentioned in the first embodiment.

Fourth Embodiment

The following will discuss a honeycomb filter according to the fourthembodiment of the present invention. Features not described below aresubstantially the same as these in the honeycomb filter according to thefirst embodiment.

The honeycomb filter according to the fourth embodiment of the presentinvention includes a plurality of honeycomb fired bodies each having anouter cell on the periphery thereof. Each honeycomb fired body includesexhaust gas emission cells each having an open end at an exhaust gasemission side and a plugged end at an exhaust gas introduction side, andexhaust gas introduction cells each having an open end at the exhaustgas introduction side and a plugged end at the exhaust gas emissionside, the exhaust gas introduction cells including first exhaust gasintroduction cells and second exhaust gas introduction cells. Thehoneycomb fired bodies are combined with one another by adhesive layersresiding therebetween. In the honeycomb filter, an oxidation catalyst issupported inside the cell walls in an amount of 5 to 60 g/L.

The exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction cell, and eachexhaust gas emission cell has the same cross sectional area as eachsecond exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction or thecells, the exhaust gas emission cells, the first exhaust gasintroduction cells, and the second exhaust gas introduction cells areeach square, and one of the sides forming the cross sectional shape ofthe first exhaust gas introduction cell faces one exhaust gas emissioncell, and none of the sides forming the cross sectional shape of thesecond exhaust gas introduction cell faces the sides forming the exhaustgas emission cell.

The cells adjacent to the outer wall include the first exhaust gasintroduction cells and the exhaust gas emission cells.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except for the plugged portionin a direction perpendicular to the longitudinal direction of the cellsthoroughly from the end at the exhaust gas introduction side to the endat the exhaust gas emission side.

In other words, the honeycomb filter according to the fourth embodimenthas substantially the same structure as the honeycomb filter accordingto the first embodiment, except that all the exhaust gas emission cells,the first exhaust gas introduction cells, and the second exhaust gasintroduction cells each have a square cross section, and also except forthe features mentioned below.

FIG. 14A is an end face view schematically illustrating one example ofthe cell arrangement in an end face of the honeycomb fired body formingthe honeycomb filter according to the fourth embodiment of the presentinvention.

In a honeycomb fired body 40 shown in FIG. 14A included in the honeycombfilter, exhaust gas emission cells 41 having a square cross section areeach adjacently surrounded fully by first exhaust gas introduction cells42 each having a square cross section and second exhaust gasintroduction cells 44 each having a square cross section across porouscell walls therebetween. The first exhaust gas introduction cells 42 andthe second exhaust gas introduction cells 44 are alternately arrangedaround each exhaust gas emission cell 41. Each second exhaust gasintroduction cell 44 has a larger cross sectional area than each firstexhaust gas introduction cell 42, and each exhaust gas emission cell 41has the same cross sectional area as each second exhaust gasintroduction cell 44.

In the cross section of the three kinds of adjacent cells, namely, theexhaust gas emission cell 41, the second exhaust gas introduction cell44, and the first exhaust gas introduction cells 42, a side 41 a facingthe first exhaust gas introduction cell 42 across a cell wall 43 amongthe sides of the square exhaust gas emission cell 41 is parallel to aside 42 a facing the exhaust gas emission cell 41 across the cell wall43 among the sides of the square first exhaust gas introduction cell 42.

Moreover, a side 42 b facing the second exhaust gas introduction cell 44across the cell wall 43 among the sides of the first exhaust gasintroduction cell 42 is parallel to a side 44 b facing the first exhaustgas introduction cell 42 across the cell wall 43 among the sides of thesecond exhaust gas introduction cell 44. Furthermore, the distancesbetween, the parallel sides of all the above pairs are the same. Thatis, the distance between the parallel sides 41 a and 42 a, and thedistance between the parallel sides 42 b and 44 b, are the same.

The square exhaust gas emission cell 41 is adjacently surrounded byalternately arranged four pieces of the first square exhaust gasintroduction cells 42 and four pieces of the second square exhaust gasintroduction cells 44 across the cell walls 43. The cross sectional areaof the second exhaust gas introduction cell 44 is larger than the crosssectional area of the first exhaust gas introduction cell 42.

Furthermore, the exhaust gas emission cells 41, the first exhaust gasintroduction cells 42, and the second exhaust gas introduction cells 44are each arranged in a manner satisfying the conditions below.

Namely, among hypothetical segments connecting geometric centers ofgravity of the square shapes of the four second exhaust gas introductioncells 44 surrounding the exhaust gas emission cell 41, an intersectionof the two segments crossing the square shape region of the exhaust gasemission cell 41 is identical with the geometric center of gravity ofthe square cross section of the exhaust gas emission cell 41.

Moreover, among the hypothetical segments connecting the geometriccenters of gravity of the square shapes of the four second exhaust gasintroduction cells 44, the four segments not crossing the square shaperegion of the exhaust gas emission cell 41 forms a square, and midpointsof the respective sides of the square are identical with the geometriccenters of gravity of the respective square shapes of the four firstexhaust gas introduction cells 42 surrounding the exhaust gas emissioncell 41.

As described above, the square exhaust gas emission cell 41 isadjacently surrounded by alternately arranged four pieces of the firstsquare exhaust gas introduction cells 42 and four pieces of the secondsquare exhaust gas introduction cells 44 across the cell walls 43 toform a single unit. The unit is two-dimensionally repeated, where thefirst exhaust gas introduction cells 42 and the second exhaust gasintroduction cells 44 in the unit are shared between adjacent cellunits, to form a honeycomb filter. The units share the first exhaust gasintroduction cells 42 and the second exhaust gas introduction cells 44.Thus, the first exhaust gas introduction cell 42 and the second exhaustgas introduction cell 44, which face the exhaust gas emission cell 41across the cell walls 43, face the exhaust gas emission cell 41 in theadjacent unit across the cell wall 43.

FIG. 15 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 15 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections shown in FIG. 15, repeated in the case where the firstexhaust gas introduction cells 42, the second exhaust gas introductioncells 44, and the exhaust gas emission cells 41 are square and theaforementioned conditions are satisfied, and also illustrates how thefirst exhaust gas introduction cells 42 and the second exhaust gasintroduction cells 44 in the unit are shared between the cell units(cell structure). A cell unit 1, a cell unit 2, and a cell unit 3 eachhave a structure in which the exhaust gas emission cell 41 is fullysurrounded by alternately arranged four pieces of the first exhaust gasintroduction cells 42 and four pieces of the second exhaust gasintroduction cells 44 across the cell walls 43 in a manner satisfyingthe aforementioned conditions.

The cell unit 2 has the same structure as that of the cell unit 1. Thecell unit 2 is adjacent to the cell unit 1 in the X direction whilesharing one piece of the first exhaust gas introduction cell 42 and twopieces of the second exhaust gas introduction cells 44 with the cellunit 1. The cells shared between the cell unit 1 and the cell unit 2 aredepicted as “shared portion 2” in FIG. 15. The cell unit 3 has the samestructure as that of the cell unit 1. The cell unit 3 is adjacent to thecell unit 1 in the Y direction while sharing one piece of the firstexhaust gas introduction cell 42 and two pieces of the second exhaustgas introduction cells 42 with the cell unit 1. The cells shared betweenthe cell unit 1 and the cell unit 3 are depicted as “shared portion 1”in FIG. 15.

Meanwhile, FIG. 15 shows four segments h, i, j, and k that do not crossthe square shape region of the exhaust gas emission cell 41, andhypothetical two segments l and m that cross the square shape region ofthe exhaust gas emission cell 41, among hypothetical segments correctionthe geometric centers of gravity of the square shapes of the four piecesof the second exhaust gas introduction cells 44. The “shared portion 2”is depicted by cross-hatching with segments in the same direction asthat of the segment m, and the “shared portion 1” is depicted bycross-hatching with segments in the same direction as that of thesegment 1.

As shown in FIG. 15, an intersection of the two segments l and m isidentical with the geometric center of gravity of the exhaust gasemission cell 41.

In the cross section of the cells, one of sides forming the crosssectional shape of the first exhaust gas introduction cell 42 faces oneexhaust gas emission cell 41. Also, the record exhaust gas introductioncell 44 and the exhaust gas emission cell 41 are arranged so that theyface each other at their corner portions. Thus, none of the sidesforming the cross sectional shape of the second exhaust gas introductioncell 44 faces the sides forming the exhaust gas emission cell 41. Thecells adjacent to an cuter wall 47 include the first exhaust gasintroduction cells 42 and the exhaust gas emission cells 41.

None of the sides forming the cross section of the second exhaust gasintroduction cell faces exhaust gas emission cells in the presentembodiment, and thus exhaust gas more easily flows into the firstexhaust gas introduction cells at an initial stage as compared to thefirst embodiment. For this reason, PMs accumulate earlier on the innercell walls of the first exhaust gas introduction cells corresponding tothe cell walls separating the first exhaust gas introduction cells andthe exhaust gas emission cells. Consequently, the aforementionedswitching of the main channel occurs further earlier. Therefore, PMstend to uniformly accumulate on the inner cell walls of the firstexhaust gas introduction cells and the inner cell walls of the secondexhaust gas introduction cells so that the pressure loss can be furtherreduced after accumulation of a certain amount of PMs. In the honeycombfilter according to the present embodiment, cracks tend not to be causedby thermal shock that occurs upon burn-removal (regeneration) of PMsaccumulated on the honeycomb filter.

In the honeycomb filter according to the present embodiment, the pluggedportions provided at the end at the exhaust gas emission side, namelythe plugged portions in the first exhaust gas introduction cells and thesecond exhaust gas introduction cells are present in vertical andhorizontal rows. Each plugged portion has a width equal to or largerthan the length of one side of the first exhaust gas introduction cell.In ordinary regeneration of honeycomb filters, heat burns firstly PMsaccumulated at the exhaust gas introduction side of a honeycomb filter.Then, burning of PMs is transferred by exhaust gas flow to the emissionside of the honeycomb filter so that all the PMs ere burned. For thisreason, positions closer to the emission side of the honeycomb filterare exposed to higher temperatures, and easily have temperaturedifference along the diameter direction of the honeycomb filter. Thus,cracks occur due to the thermal stress. Such cracks are prominent in thecase where the cross sectional, areas are different among the cells, andalso in the case where the cross sections of the exhaust gasintroduction cells where PMs accumulate have square cross section(s). Incontrast, in the honeycomb filter according to the embodiment of thepresent invention, the plugged portions aligned in vertical andhorizontal rows at the exhaust gas emission side function as thermallyconductive layers and as layers for dissipating heat to outside. Thus,the temperature difference is small along the diameter direction of thehoneycomb filter. This reduces the thermal stress to be caused toprevent occurrence of cracks. Moreover, in the honeycomb filteraccording to the embodiment of the present invention, the amount of theplug in the second exhaust gas introduction cells, in which a largeramount of PMs accumulate compared with the first exhaust gasintroduction cells, is large due to the size of the cross sectionalarea, and thereby the thermal capacity of the plugged portions increase.Such plugged portions can prevent an increase in the temperature of thesecond exhaust gas introduction cells, where the heat of burning PMs ishigher, even in consideration of each cell. This reduces the temperaturedifference in the diameter direction of the honeycomb filter, therebyreducing the thermal stress to be caused.

Thus, the honeycomb filter according to the present embodiment canprevent cracks during the regeneration even though it includes exhaustgas introduction cells having square cross sections.

The cross sectional area of each first exhaust gas introduction cell 42is preferably 20 to 50% the size, and more preferably 22 to 45% the sizeof the cross sectional area of each second exhaust gas introduction cell44.

In a honeycomb fired body shown in FIG. 14A, the cross sectional area ofeach exhaust gas emission cell 41 is equal to the cross sectional areaof each second exhaust gas introduction cell 44; however, the crosssectional area of each exhaust gas emission cell 41 may be larger thanthe cross sectional area of each second exhaust gas introduction cell44.

The cross sectional area of each exhaust gas emission cell 41 ispreferably 1.05 to 1.5 times the size of the cross sectional area ofeach second exhaust gas introduction cell 44.

The following describes the thickness of the cell walls in the crosssection of the honeycomb fired body 40 included in the honeycomb filteraccording to the fourth embodiment based on the aforementioneddefinition of the cell walls. Supposing that a straight line Z₄₂connecting the center of gravity O₄₁ of the exhaust gas emission cell 41and the center of gravity O₄₂ of the first exhaust gas introduction cell42 is given, the thickness of the cell wall 43 at a part overlapped withthe straight line Z₄₂ (the thickness between the side 42 a and the side41 a) is determined as thickness X₁. Supposing that a straight line Z₄₄connecting the center of gravity O₄₄ of the second exhaust gasintroduction cell 44 and the center of gravity O₄₁ of the exhaust gasemission cell 41 is given, the thickness of the cell wall 43, whichseparates the second exhaust gas introduction cell 44 and the exhaustgas emission cell 41, at a part overlapped with the straight line Z₄₄(the length between a corner portion 44 c of the second exhaust gasintroduction cell 44 and a corner portion 41 c of the exhaust gasemission cell 41) is determined as thickness Y₁.

The thickness of the cell walls of the honeycomb filter varies dependingon the parts as shown in FIG. 14A; however, the thicknesses, includingthe thickness X₁ and the thickness Y₁, may be set to be within a rangeof 0.10 to 0.46 mm.

The following will discuss a modified example of the honeycomb filteraccording to the fourth embodiment of the present invention.

FIG. 14B is an end face view illustrating one modified example of thehoneycomb fired body forming the honeycomb filter according to thefourth embodiment of the present invention.

In a honeycomb fired body 40 a shown in FIG. 14B included in thehoneycomb filter, the shape of an exhaust gas emission cell 41A adjacentto an outer wall 47 a is, compared to the shape of an exhaust gasemission cell 41 not adjacent to the outer wall 47 a, partially deformedto be rectangle in accordance with the line along the inner wall, whichforms the outer wall, in the first exhaust gas introduction cells 42Aadjacent to the outer wall 47 a. An exhaust gas emission cell 41B at thecorner portion has a square shape with a smaller cross sectional areacompared to the exhaust gas emission cell 41 not adjacent to the outerwall 47 a.

With such shapes of the cells, the boundary between the outer wall 47 aand the exhaust gas emission cells 41A and 41B and the first exhaust gasintroduction cells 42A adjacent to the outer wall 47 a is formedlinearly, and the thickness of the outer wall 47 a is uniform.

The following will discuss another modified example of the honeycombfilter according to the fourth embodiment of the present invention.

FIG. 14C is an end face view illustrating another modified example ofthe honeycomb fired body forming the honeycomb filter according to thefourth embodiment of the present invention.

In a honeycomb fired body 40 b shown in FIG. 14C included in thehoneycomb filter, all the first exhaust gas introduction cells have thesame shape, and all the exhaust gas emission cells 41 have the sameshape. A side 470 b corresponding to an outer periphery of an outer wall47 b changes in accordance with the shapes of the first exhaust gasintroduction cells 42 and the exhaust gas emission cells 41 adjacent tothe outer wall 47 b. The thickness of the outer wall 47 b is uniform.

In other words, to make the thickness of the outer wall 47 b uniform,the side 470 b corresponding to the outer periphery of the outer wall 47b becomes rugged along with the shapes of the first exhaust gasintroduction cells 42 and the exhaust gas emission cells 41 adjacent tothe outer wall 47 b.

The features of the embodiment other than those mentioned above are thesame as the features described in relation to the first embodiment, andthus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can bemanufactured in the same manner as in the first embodiment of thepresent invention, except that a die having a different shape is used inthe extrusion molding process.

In the honeycomb filter according to the present embodiment, unlike thefirst embodiment, the exhaust gas emission cells 41 and the secondexhaust gas introduction cells 44 have square cross sections, and allthe sides forming the cross sectional shape of the second exhaust gasintroduction cells 44 do not face the exhaust gas emission cells 41.Among the sides forming the cross sectional shape of the first exhaustgas introduction cells 42, a side 42 a faces one of the exhaust gasemission cells 41. Thus, like the honeycomb filter according to thefirst embodiment, it is considered that exhaust gas easily flows intothe first exhaust gas introduction cells 42 at an initial stage. Afteraccumulation of a certain amount of PMs, exhaust gas tends to flows intothe second exhaust gas introduction cells 44.

The honeycomb filter according to the present embodiment issubstantially the same as the honeycomb filter 20 according to the firstembodiment concerning basic arrangement of cells, manner of plugging,size difference among the cross sectional areas of the cells, or thelike. Thus, the honeycomb filter according to the present embodiment canexert the same effects as the effects (1) to (3), (5) and (7) to (10)described in relation to the first embodiment.

Fifth Embodiment

The following will discuss a honeycomb filter according to the fifthembodiment of the present invention. Features not described below aresubstantially the same as those in the honeycomb filter according to thefirst embodiment.

The honeycomb filter according to the fourth embodiment of the presentinvention includes a plurality of honeycomb fired bodies each having anouter wall on the periphery thereof. Each honeycomb fired body includesexhaust gas emission cells each having an open end at an exhaust gasemission side and a plugged end at an exhaust gas introduction side, andexhaust gas introduction cells each having an open end at the exhaustgas introduction side and a plugged end at the exhaust gas emissionside, the exhaust gas introduction cells including first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga larger cross sectional area than each first exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells. The honeycomb fired bodies are combined with one another byadhesive layers residing therebetween. In the honeycomb filter, anoxidation catalyst is supported inside the cell walls in an amount of 5to 60 g/L.

In the honeycomb filter according to the fifth embodiment of the presentinvention, in the cross section perpendicular to the longitudinaldirection of the cells, the cross sectional area of each exhaust gasemission cell is equal in size to the cross sectional area of eachsecond exhaust gas introduction cell. The exhaust gas emission cells andthe exhaust gas introduction cells are each in a shape formed by acurved line in the cross section perpendicular to the longitudinaldirection of the cells, and all the exhaust gas emission cells, thefirst exhaust gas introduction cells, and the second exhaust gasintroduction cells have round cross sectional shapes.

The cells adjacent to the outer wall include the first exhaust gasintroduction cells and the exhaust gas emission cells.

The honeycomb filter according ho the fifth embodiment of the presentinvention has substantially the same features as the features of thehoneycomb filter according to the first embodiment of the presentinvention, except that the cross sectional shapes of the exhaust gasemission cells, the second exhaust gas introduction cells, and the firstexhaust gas introduction cells are different in the directionperpendicular to the longitudinal direction of the cells.

FIG. 16 is an end face view schematically illustrating one example ofthe cell arrangement in an end face of a honeycomb fired body formingthe honeycomb filter according to a fifth embodiment of the presentinvention.

A honeycomb fired body 50 in a honeycomb filter according to the fifthembodiment of the present invention includes exhaust gas emission cells51, first exhaust gas introduction cells 52, cell walls 53, and secondexhaust gas introduction cells 54.

The exhaust gas emission cells 51 are each adjacently surrounded fullyby the first exhaust gas introduction cells 52 and the second exhaustgas introduction cells 54 across porous cell walls 53 residingtherebetween. The cells adjacent to the outer wall include the exhaustgas emission cells 51 and the first exhaust gas introduction cells 52.

In the honeycomb fired body shown in FIG. 16, in the cross sectionperpendicular to the longitudinal direction of the cells, the crosssectional area of each second exhaust gas introduction cell 54 is equalin size to the cross sectional area of each exhaust gas emission cell51, and the cross sectional area of each first exhaust gas introductioncells 52 is smaller than the cross sectional area of each second exhaustgas introduction cell 54. The cross sectional area of each first exhaustgas introduction cell 52 is preferably 20 to 50% the size of the crosssectional area of each second exhaust gas introduction cell 54.

All the exhaust gas emission cells 51, the first exhaust gasintroduction cells 52, and the second exhaust gas introduction cells 54have round cross sectional shapes.

The following describes the thickness of the cell walls in the crosssection of the honeycomb fired body 50 shown in FIG. 16 included in thehoneycomb filter according to the fifth embodiment based on theaforementioned definition of the cell walls. Supposing that a straightline Z₅₂ connecting the center of gravity O₅₁ of the exhaust gasemission cell 51 and the center of gravity O₅₂ of the first exhaust gasintroduction cell 52 is given, the thickness of the cell wall 53 at apart overlapped with the straight line is determined as thickness X₂.Supposing that a straight line Z₅₄ connecting the center of gravity O₅₄of the second exhaust gas introduction cell 54 and the center of gravityO₅₁ of the exhaust gas emission cell 51 is given, the thickness of thecell wall 53, which separates the second exhaust gas introduction cell54 and the exhaust gas emission cell 51, at a part overlapped with thestraight line Z₅₄ determined as thickness Y₂.

In the honeycomb fired body 50 according to the present embodiment, thethickness X₂ of the cell wall 53 separating the first exhaust gasintroduction cell 52 and the exhaust gas emission cell 51 is smallerthan the thickness Y₂ of the cell wall 53 separating second exhaust gasintroduction cell 54 and the exhaust gas emission cell 51.

This embodiment provides a shape that can have a larger differencebetween the thickness of the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells and the thicknessof the cell walls separating the second exhaust gas introduction cellsand the exhaust gas emission cells. For this reason, the exhaust gasmore easily flows into the first exhaust gas introduction cells at aninitial stage as compared to the first embodiment. Thus, PMs accumulateearlier on the inner cell walls of the first exhaust gas introductioncells corresponding to the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells. Consequently, theaforementioned switching of the main channel occurs further earlier.Therefore, PMs tend to uniformly accumulate on the inner cell walls ofthe first exhaust gas introduction cells and the inner cell walls of thesecond exhaust gas introduction cells so that the pressure loss can befurther reduced after accumulation of a certain amount of PMs.

The thickness X₂ of the cell wall 53 separating the first exhaust gasintroduction cell 52 and the exhaust gas emission cell 51 is preferably40 to 75% the thickness Y₂ of the cell wall 53 separating the secondexhaust gas introduction cell 54 and the exhaust gas emission cell 51.

In the honeycomb fired body 50 according to the present embodiment, thethickness of the cell wall 53 separating the first exhaust gasintroduction cell 52 and the second exhaust gas introduction cell 54 maybe determined in the same manner as the thickness of the cell wall 53separating the first exhaust gas introduction cell 52 and the exhaustgas emission cell 51.

In the honeycomb fired body 50 shown in FIG. 16, the thickness of thecell wall 53 separating the first exhaust gas introduction cell 52 andthe second exhaust gas introduction cell 54 is smaller than thethickness of the cell wall 53 separating the first exhaust gasintroduction cell 52 and the exhaust gas emission cell 51.

In the honeycomb filter according to the fifth embodiment of the presentinvention, as shown in FIG. 16, the shape of each first exhaust gasintroduction cell adjacent to the outer wall may be the same as that ofeach first exhaust gas introduction cell not adjacent to the outer wall,and the shape of each exhaust gas emission cell adjacent to the outerwall may be the same as that of each exhaust gas emission cell notadjacent to the outer wall. Alternatively, the exhaust gas emission cellmay have a side partially deformed in accordance with the lineconnecting the outermost points on the inner walls of the first exhaustgas introduction cells adjacent to the outer walls so that the thicknessof the outer wall except for the corner portions is substantiallyuniform.

In the honeycomb filter according to the fifth embodiment of the presentinvention, the thickness of the outer wall may be uniform in accordancewith the shapes of the first exhaust gas introduction cells and theexhaust gas emission cells adjacent to the outer wall, and the shapes ofthe first exhaust gas introduction cells and the exhaust gas emissioncells adjacent to the outer wall may be the same as the exhaust gasemission cells not adjacent to the outer wall, respectively. In otherwords, the outer wall is bending in accordance with the shapes of thefirst exhaust gas introduction cells and the exhaust gas emission cellsto maintain the uniform thickness in this case.

The features of the embodiment other than those mentioned above are thesame as the features described in relation to the first embodiment, andthus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can bemanufactured in the same manner as in the first embodiment of thepresent invention, except that a die having a different shape is used inthe extrusion molding process.

Hereinafter, the effects of the honeycomb filter according to the fifthembodiment of the present invention are listed.

The honeycomb filter described in the first embodiment of the presentinvention has a feature that the side 12 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe first exhaust gas introduction cell 12 is longer than the side 14 afacing the exhaust gas emission cell 11 among the sides forming thecross sectional shape of the second exhaust gas introduction cell 14.Meanwhile, the honeycomb filter according to the fourth embodiment has afeature that all the exhaust gas emission cells, the first exhaust gasintroduction cells, and the second exhaust gas introduction cells haveround cross sectional shapes, and the thickness of the cell wallsseparating the first exhaust gas introduction cells and the exhaust gasemission cells is smaller than the thickness of the cell wallsseparating the second exhaust gas introduction cells and the exhaust gasemission cells. The fifth embodiment is different on the above pointsfrom the first embodiment. Other features are substantially the same.

A smaller thickness of the cell walls may lead to easier passage ofexhaust gas through the cell walls so that the pressure loss is reducedeven in the case where all the exhaust gas emission cells, the firstexhaust gas introduction cells, and the second exhaust gas introductioncells have round cross sectional shapes. Thus, the length of sidesforming cross sections of the cells may be considered to correspond tothe thickness of the cell walls separating the cells. Hence, thehoneycomb filter according to the fourth embodiment of the presentinvention can exert the same effects as the effects (1) to (3), (5) and(7) to (10) described in relation to the first embodiment.

Sixth Embodiment

The following will discuss a honeycomb filter according to the sixthembodiment of the present invention. Features not described below aresubstantially the same as those in the honeycomb filter according to thefirst embodiment.

The honeycomb filter according to the sixth embodiment of the presentinvention includes a plurality of honeycomb fired bodies each having anouter wall on the periphery thereof. Each honeycomb fired body includesexhaust gas emission cells each having an open end at an exhaust gasemission side and a plugged end at an exhaust gas introduction side, andexhaust gas introduction cells each having an open end at the exhaustgas introduction side and a plugged end at the exhaust gas emissionside, the exhaust gas introduction cells including first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga larger cross sectional area than each first exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells. The honeycomb fired bodies are combined with one another byadhesive layers residing therebetween. In the honeycomb filter, anoxidation catalyst is supported inside the cell walls in an amount of 5to 60 g/L.

In the honeycomb filter according to the sixth embodiment of the presentinvention, in the cross section perpendicular to the longitudinaldirection of the cells, the cross sectional area of the exhaust gasemission cells is equal in size to the cross sectional area of thesecond exhaust gas introduction cells. In the cross sectionperpendicular to the longitudinal direction of the cells, the exhaustgas emission cells and the exhaust gas introduction cells are each in ashape formed by carved lines; and the exhaust gas emission cells and thesecond exhaust gas introduction cells each have a convex square crosssection formed by four outwardly curved lines, whereas the first exhaustgas introduction cells each have a concave square cross section formedby four inwardly curved lines. The cells adjacent to the outer wallinclude the exhaust gas emission cells and the first exhaust gasintroduction cells.

The honeycomb filter according to the sixth embodiment of the presentinvention has substantially the same features as the features of thehoneycomb filter according to the first embodiment of the presentinvention, except that the cross sectional shapes of the exhaust gasemission cells, the second exhaust gas introduction cells, and the firstexhaust gas introduction cells are different in the directionperpendicular to the longitudinal direction of the cells.

FIG. 17 is an end face view schematically illustrating one example ofthe cell arrangement in an end face of a honeycomb fired body formingthe honeycomb filter according to the sixth embodiment of the presentinvention.

A honeycomb fired body 60 included in a honeycomb filter according tothe sixth embodiment of the present invention includes exhaust gasemission cells 61, first exhaust gas introduction cells 62, cell walls63, and second exhaust gas introduction cells 64. The exhaust gasemission cells 61 are each adjacently surrounded fully by the firstexhaust gas introduction cells 62 and the second exhaust gasintroduction cells 64 across porous cell walls 63 residing therebetween.

In the honeycomb fired body shown in FIG. 17, in the cross sectionperpendicular to the longitudinal direction of the cells, the crosssectional area of each second exhaust gas introduction cell 64 is equalin site to the cross sectional area of each exhaust gas emission cell61, and the cross sectional area of each first exhaust gas introductioncell 62 is smaller than the cross sectional area of each second exhaustgas introduction cell 64. The cross sectional area of each first exhaustgas introduction cell 62 is preferably 20 to 50% the size of the crosssectional area of each second exhaust gas introduction cell 64.

The exhaust gas emission cells 61 and the second exhaust gasintroduction cells 64 each have a convex square cross section formed byfour outwardly curved lines.

FIG. 18A is an explanatory diagram schematically illustrating oneexample of the convex square cell shape. FIG. 18B is an explanatorydiagram schematically illustrating one example of the concave squarecell shape. FIG. 18C is an explanatory diagram schematicallyillustrating one example of the concave square shape in which a vertexportion is chamfered. FIG. 18D is an explanatory diagram schematicallyillustrating one example of the convex square shape in which a vertexportion is chamfered.

FIG. 18A shows a second exhaust gas introduction cell 64 having a convexsquare cross section, and a square 65 reread by connecting four vertices64 e of the second exhaust gas introduction cell 64.

In the explanation of the embodiments of the present invention, theconvex square refers to a figure that is substantially square havingfour carved sides. The sides are curved outwardly from the square formedby connecting the four vertices of the substantially square figure.

FIG. 18A shows that the sides 64 a forming the cross section of thesecond exhaust gas introduction cell 64 are curved (convex) outwardlyfrom the geometric center of gravity of the convex square toward outsidethe square 65.

Although FIG. 18A illustrates the cross section of the second exhaustgas introduction cell 64 as an example of the convex square cell shape,the cross section of the exhaust gas emission cell 61 is substantiallythe same as the cross section of the second exhaust gas introductioncell 64.

The first exhaust gas introduction cells 62 each have a concave squarecross section formed by four inwardly curved lines.

FIG. 18B shows a first exhaust gas introduction cell 62 having a concavesquare cross section, and a square 66 formed by connecting four vertices62 e of the first exhaust gas introduction cell 62.

In the explanation of the embodiments of the present invention, theconcave square refers to a figure that is substantially square havingfour curved sides. The sides are curved (concaved) inwardly from thesquare formed by connecting the four vertices of the substantiallysquare figure toward the geometric cancer of gravity of the concavesquare.

FIG. 18B shows that the sides 62 a forming the cross section of thefirst exhaust gas introduction cell 62 are curved (concaved) from thesquare 66 towards the geometric center of gravity of the concave square.

According to the present embodiment, the first exhaust gas introductioncells each have an acute angle portion that causes resistance byinhibiting gas flow, whereas the second exhaust gas introduction cellseach have obtuse angles that allow easy gas flow. Thus, in comparison tothe first embodiment, after only a small amount of PMs are accumulatedon the inner cell walls separating the second exhaust gas introductioncells and the exhaust gas emission cells, exhaust gas easily flows intothe first exhaust gas introduction cells. Therefore, PMs tend touniformly accumulate on the inner walls of the first exhaust gasintroduction cells and the inner walls of the second exhaust gasintroduction cells so that the pressure loss after accumulation of acertain amount of PMs can be further reduced.

In the explanation of the embodiments of the present invention, theconvex square and the concave square include shapes that are chamferedin the vicinity of the vertex portions thereof.

FIG. 18C shows a shape in which a side 62 a 1 and a side 62 a 2, whichare curved lines forming the concave square, are not directly connected,and the side 62 a 1 is connected to the side 62 a 2 via a chamferedportion 62 b that is chamfered with a straight line.

In the case where the curved sides forming the concave square areconnected via the chamfered portion, an intersection 62 c ofhypothetical curved lines extended respectively from the side 62 a 1 andthe side 62 a 2 as shown by a dotted line in FIG. 18C is defined as avertex.

FIG. 18D shows a shape in which a side 64 a 1 and a side 64 a 2, whichare curved lines forming the convex square, are not directly connectedeach other, and the side 62 a 1 is connected to the side 62 a 2 via achamfered portion 64 b that is chamfered by a straight line.

In the case where the curved sides forming the convex square areconnected each other via the chamfered portion, an intersection 64 c ofhypothetical curved lines extended respectively from the side 64 a 1 andthe side 64 a 2 as shown by a dotted line in FIG. 18D is defined as avertex.

Whether the cross section formed by curved lines is a convex square or aconcave square can be determined by hypothetically depicting a square byconnecting the vertices intersections 62 c or intersections 64 c).

The chamfered portion is not limited to the one chamfered with astraight line, but may be one chamfered with a curved line.

In the honeycomb filter of the present embodiment, the thickness of thecell walls 63 between the first exhaust gas introduction cells 62 andthe exhaust gas emission cells 61 is smaller than the thickness of thecell walls 63 between the second exhaust gas introduction cells 64 andthe exhaust gas emission cells 61.

The following describes the thickness of the cell walls in the crosssection of the honeycomb fired body 60 according to the sixth embodimentshown in FIG. 17 based on the aforementioned definition of the cellwalls. Supposing that a straight line Z₆₂ connecting the center ofgravity O₆₁ of the exhaust gas emission cell 61 and the center ofgravity O₆₂ of the first exhaust gas introduction cell 62 is given, thethickness of the cell wall 63 at a part overlapped with the straightline (thickness between the side 62 a and the side 61 a) is determinedas thickness X₃. Supposing that a straight line Z₆₄ connecting thecenter of gravity O₆₄ of the second exhaust gas introduction cell 64 andthe center of gravity O₆₁ of the exhaust gas emission cell 61 is given,a thickness of the cell wall 63, which separates the second exhaust gasintroduction cell 64 and the exhaust gas emission cell 61, at a partoverlapped with the straight line Z₆₄ (distance between a vertex 64 e ofthe second exhaust gas introduction cell 64 and a vertex 61 e of theexhaust gas emission cell 61) is determined as thickness Y₃.

In the honeycomb fired body 60 according to the present embodiment, thethickness X₃ of the cell wall 63 separating the first exhaust gasintroduction cell 62 and the exhaust gas emission cell 61 is smallerthan the thickness Y₃ of the cell wall 63 separating the second exhaustgas introduction cell 64 and the exhaust gas emission cell 61.

In the honeycomb fired body 60 according to the present embodiment, thethickness of the cell wall separating the first exhaust gas introductioncell 62 and the second exhaust gas introduction cell 64 may bedetermined in the same manner as the thickness of the cell wallseparating the first exhaust gas introduction cell 62 and the exhaustgas emission cell 61.

In the honeycomb fired body 60 shown in FIG. 17, the thickness of thecell wall 63 separating the first exhaust gas introduction cell 62 andthe second exhaust gas introduction cell 64 is uniform, and is equal tothe thickness of the cell wall 63 separating the first exhaust gasintroduction cell 62 and the exhaust gas emission cell 61.

In the honeycomb filter according to the sixth embodiment of the presentinvention, the thickness of the outer wall may be uniform, the shape ofthe first exhaust gas introduction cells adjacent to the outer wall mayhave the same shape as the first exhaust gas introduction cells notadjacent to the outer wall, and the exhaust gas emission cell adjacentto the outer wall may have the same shape as the exhaust gas emissioncell not adjacent to the outer wall. With an aim of making the thicknessof the outer wall substantially uniform, except for the corner portions,a side adjacent to the outer wall of the exhaust gas emission cell maybe partially deformed along with the line connecting the outermostpoints on the inner walls of the first exhaust gas introduction cellsadjacent to the outer wall.

In the honeycomb filter according to the sixth embodiment of the presentinvention, the thickness of the outer wall may be uniform along with theshape of the exhaust gas introduction cells adjacent to the outer wall,and the shapes of the first exhaust gas introduction cells and theexhaust gas emission cells adjacent to the outer wall may be tire sameas those of the first exhaust gas introduction cells and the exhaust gasemission cells not adjacent to the outer wall, respectively. In otherwords, the outer wall is bending in accordance with the shapes of thefirst exhaust gas introduction cells and the exhaust gas emission cellsto maintain the uniform thickness in this case.

The honeycomb filter according to the present embodiment can bemanufactured in the same manner as in the first embodiment of thepresent invention, except that a die having a different shape is used inthe extrusion molding process.

Hereinafter, the effects of the honeycomb filter according to the sixthembodiment of the present invention are listed.

The honeycomb filter described in the first embodiment of the presentinvention has a feature that the side 12 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe first exhaust gas introduction cell 12 is larger than the side 14 afacing the exhaust gas emission cell 11 among the sides forming thecross sectional shape of the second exhaust gas introduction cell 14.Meanwhile, the honeycomb filter according to the sixth embodiment has afeature that the thickness of the cell walls separating the firstexhaust gas introduction cells and the exhaust gas emission cells issmaller than the thickness of the cell walls separating the secondexhaust gas introduction cells and the exhaust gas emission cells. Thesixth embodiment is different on the above points from the firstembodiment. Other features are substantially the same.

A smaller thickness of the cell walls may lead to easier passage ofexhaust gas through the cell walls so that the pressure loss may bereduced. Thus, the length of the sides forming the cross sections of thecells may correspond to the thickness of the cell walls separating thecells. Hence, the honeycomb filter according to the sixth embodiment ofthe present invention can exert the same effects as the effects (1) to(3), (5), and (7) to (10) described in relation to the first embodiment.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The invention claimed is:
 1. A honeycomb filter comprising: a pluralityof cells through which exhaust gas is to flow and which include exhaustgas introduction cells and exhaust gas emission cells, the exhaust gasintroduction cells each having an open end at an exhaust gasintroduction side and a plugged end at an exhaust gas emission side, theexhaust gas emission cells each having an open end at the exhaust gasemission side and a plugged end at the exhaust gas introduction side;porous cell walls defining rims of the plurality of cells; an oxidationcatalyst supported inside the porous cell walls in an amount of 5 to 60g/L; the exhaust gas introduction cells and the exhaust gas emissioncells each having a uniform cross sectional shape except for a pluggedportion in a cross section perpendicular to a longitudinal direction ofthe plurality of cells thoroughly from the exhaust gas introduction sideto the exhaust gas emission side; the exhaust gas emission cells havingan average cross sectional area larger than an average cross sectionalarea of the exhaust gas introduction cells in the cross sectionperpendicular to the longitudinal direction; and a total volume of theexhaust gas introduction cells being larger than a total volume of theexhaust gas emission cells, wherein each of the exhaust gas emissioncells is adjacently surrounded fully by the exhaust gas introductioncells across the porous cell walls, wherein the exhaust gas introductioncells include first exhaust gas introduction cells and second exhaustgas introduction cells, wherein each of the second exhaust gasintroduction cells has a cross sectional area larger than a crosssectional area of each of the first exhaust gas introduction cells inthe cross section perpendicular to the longitudinal direction of theplurality of cells, wherein each of the exhaust gas emission cells has across sectional area equal to or larger than the cross sectional area ofeach of the second exhaust gas introduction cells in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas introduction cellsand the exhaust gas emission cells each have a polygonal shape, andwherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, a side forming a cross sectionalshape of each of the first exhaust gas introduction cells faces one ofthe exhaust gas emission cells, a side forming a cross sectional shapeof each of the second exhaust gas introduction cells faces one of theexhaust gas emission cells, and the side of each of the first exhaustgas introduction cells is longer than the side of each of the secondexhaust gas introduction cells, or a side forming a cross sectionalshape of each of the first exhaust gas introduction cells faces one ofthe exhaust gas emission cells, and none of sides forming a crosssectional shape of each of the second exhaust gas introduction cellsfaces the exhaust gas emission cells.
 2. The honeycomb filter accordingto claim 1, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, length of the sideforming the cross sectional shape of each of the second exhaust gasintroduction cells is not more than 0.8 times length of the side formingthe cross sectional shape of each of the first exhaust gas introductioncells.
 3. The honeycomb filter according to claim 1, wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas emission cells are each octagonal,the first exhaust gas introduction cells are each square, and the secondexhaust gas introduction cells are each octagonal.
 4. The honeycombfilter according to claim 3, wherein the porous cell walls separatingthe plurality of cells have a uniform thickness in any part of thehoneycomb filter.
 5. The honeycomb filter according to claim 3, wherein,in the cross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas emission cells have octagonal crosssections, the first exhaust gas introduction cells have square crosssections, and the second exhaust gas introduction cells have octagonalcross sections, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the cross sectionalshape of each of the second exhaust gas introduction cells is congruentwith a cross sectional shape of each of the exhaust gas emission cells,and wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas emission cells, thefirst exhaust gas introduction cells, and the second exhaust gasintroduction cells are arranged in a manner that the exhaust gasemission cells are each surrounded by alternately arranged four piecesof the first exhaust gas introduction cells and four pieces of thesecond exhaust gas introduction cells across the porous cell walls,provided that hypothetical segments connecting geometric centers ofgravity of the octagonal cross sections of the four pieces of the secondexhaust gas introduction cells surrounding a reference exhaust gasemission cell of the exhaust gas emission cells are given, anintersection of two segments crossing the reference exhaust gas emissioncell is identical with a geometric center of gravity of an octagonalcross section of the reference exhaust gas emission cell, and foursegments not crossing the reference exhaust gas emission cell form asquare, and midpoints of respective sides of the square are identicalwith geometric centers of gravity of the square cross sections of thefour pieces of the first exhaust gas introduction cells surrounding thereference exhaust gas emission cell, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells, aside facing one of the first exhaust gas introduction cells across afirst cell wall among sides forming the cross sectional shape of one ofthe exhaust gas emission cells is parallel to a side facing one of theexhaust gas emission cells across the first cell wall among sidesforming the cross sectional shape of one of the first exhaust gasintroduction cells, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, a side facing one ofthe second exhaust gas introduction cells across a second cell wallamong sides forming the cross sectional shape of one of the exhaust gasemission cells is parallel to a side facing one of the exhaust gasemission cells across the second cell wall among sides forming the crosssectional shape of one of the second exhaust gas introduction cells,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, a side facing one of the secondexhaust gas introduction cells across a cell wall among sides formingthe cross sectional shape of one of the first exhaust gas introductioncells is parallel to a side facing one of the first exhaust gasintroduction cells across the cell wall among sides forming the crosssectional shape of one of the second exhaust gas introduction cells, andwherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, distances between parallel sidesare the same.
 6. The honeycomb filter according to claim 1, wherein, inthe cross section perpendicular to the longitudinal direction of theplurality of cells, the cross sectional area of each of the secondexhaust gas introduction cells is equal in size to the cross sectionalarea of each of the exhaust gas emission cells, and wherein the crosssectional area of each of the first exhaust gas introduction cells is 20to 50% size of the cross sectional area of each of the second exhaustgas introduction cells.
 7. The honeycomb filter according to claim 1,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas emission cells, thefirst exhaust gas introduction cells, and the second exhaust gasintroduction cells are all square.
 8. The honeycomb filter according toclaim 7, wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the cross sectional area of each ofthe second exhaust gas introduction cells is equal in size to the crosssectional area of each of the exhaust gas emission cells, and wherein,in the cross section perpendicular to the longitudinal direction of theplurality of cells, the cross sectional area of each of the firstexhaust gas introduction cells is 20 to 50% size of the cross sectionalarea of each of the second exhaust gas introduction cells.
 9. Thehoneycomb filter according to claim 7, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,the exhaust gas emission cells have square cross sections, the firstexhaust gas introduction cells have square cross sections, and thesecond exhaust gas introduction cells have square cross sections,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the cross sectional shape of eachof the second exhaust gas introduction cells is congruent with a crosssectional shape of each of the exhaust gas emission cells, wherein, inthe cross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas emission cells, the first exhaustgas introduction cells, and the second exhaust gas introduction cellsare arranged in a manner that the exhaust gas emission cells are eachsurrounded by alternately arranged four pieces of the first exhaust gasintroduction cells and four pieces of the second exhaust gasintroduction cells across the porous cell walls, provided thathypothetical segments connecting geometric centers of gravity of thesquare cross sections of the four pieces of the second exhaust gasintroduction cells surrounding a reference exhaust gas emission cell ofthe exhaust gas emission cells are given, an intersection of the twosegments crossing the reference exhaust gas emission cell is identicalwith a geometric center of gravity of a square cross section of thereference exhaust gas emission cell, and four segments not crossing thereference exhaust gas emission cell form a square, and midpoints ofrespective sides of the square are identical with geometric centers ofgravity of the square cross sections of the four pieces of the firstexhaust gas introduction cells surrounding the reference exhaust gasemission cell, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, a side facing one ofthe first exhaust gas introduction cells across a first cell wall amongsides forming the cross sectional shape of one of the exhaust gasemission cells is parallel to a side facing one of the exhaust gasemission cells across the first cell wall among sides forming the crosssectional shape of one of the first exhaust gas introduction cells,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, a side facing one of the secondexhaust gas introduction cells across a cell wall among sides formingthe cross sectional shape of one of the first exhaust gas introductioncells is parallel to a side facing one of the first exhaust gasintroduction cells across the cell wall among sides forming the crosssectional shape of one of the second exhaust gas introduction cells, andwherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, distances between parallel sidesare the same.
 10. The honeycomb filter according to claim 1, wherein, inthe cross section perpendicular to the longitudinal direction of theplurality of cells, vertex portions of the polygonal shape are formed bycurved lines.
 11. The honeycomb filter according to claim 1, wherein, inthe cross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas emission cells, the first exhaustgas introduction cells, and the second exhaust gas introduction cellsare point-symmetrical polygons each having not more than eight sides.12. The honeycomb filter according to claim 1, wherein the exhaust gasintroduction cells consist only of the first exhaust gas introductioncells, and the second exhaust gas introduction cells each having thecross sectional area larger than the cross sectional area of each of thefirst exhaust gas introduction cells in the cross section perpendicularto the longitudinal direction of the plurality of cells.
 13. Thehoneycomb filter according to claim 1, wherein the honeycomb filtercomprises a plurality of honeycomb fired bodies, wherein each of theplurality of honeycomb fired bodies has the exhaust gas emission cells,the first exhaust gas introduction cells, and the second exhaust gasintroduction cells, wherein each of the plurality of honeycomb firedbodies has an outer wall on an outer periphery of each of the pluralityof honeycomb fired bodies, and wherein the plurality of honeycomb firedbodies are combined with one another by adhesive layers residing betweenthe plurality of honeycomb fired bodies.
 14. The honeycomb filteraccording to claim 13, wherein the outer wall has corner portions, andwherein a side, which contacts the outer wall, of each of the exhaustgas introduction cells and the exhaust gas emission cells adjacent tothe outer wall is straight and parallel to a side corresponding to anouter periphery of the outer wall in a manner that a thickness of theouter wall is uniform except for the corner portions in the crosssection perpendicular to the longitudinal direction of the plurality ofcells.
 15. The honeycomb filter according to claim 1, wherein thehoneycomb filter comprises honeycomb fired bodies, and wherein thehoneycomb fired bodies include one of silicon carbide andsilicon-containing silicon carbide.
 16. The honeycomb filter accordingto claim 1, wherein the porous cell walls have a thickness of 0.10 to0.46 mm.
 17. The honeycomb filter according to claim 1, wherein theporous cell walls have pores having an average pore diameter of 8 to 25μm.
 18. The honeycomb filter according to claim 1, wherein the porouscell walls have a porosity of 40 to 70%.
 19. The honeycomb filteraccording to claim 1, further comprising: a periphery coat layerprovided on a periphery of the honeycomb filter.
 20. The honeycombfilter according to claim 1, wherein, in the cross sectional shapeperpendicular to the longitudinal direction of the plurality of cells,the first exhaust gas introduction cells, the second exhaust gasintroduction cells, and the exhaust gas emission cells each have auniform cross sectional shape except for the plugged portion in thecross section perpendicular to the longitudinal direction of theplurality of cells thoroughly from the exhaust gas introduction side tothe exhaust gas emission side, wherein, in the cross sectional shapeperpendicular to the longitudinal direction of the plurality of cells,the cross sectional shape of each of the first exhaust gas introductioncells is different from the cross sectional shape of each of the secondexhaust gas introduction cells, and wherein, in the cross sectionalshape perpendicular to the longitudinal direction of the plurality ofcells, the cross sectional shape of each of the exhaust gas emissioncells is different from the cross sectional shape of each of the firstexhaust gas introduction cells.
 21. The honeycomb filter according toclaim 1, wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, a cell unit is two-dimensionallyrepeated in a manner that the first exhaust gas introduction cells andthe second exhaust gas introduction cells surrounding each of theexhaust gas emission cells in the cell unit are shared between adjacentcell units, wherein the cell unit has a cell structure such that each ofthe exhaust gas emission cells is adjacently surrounded fully by theexhaust gas introduction cells across the porous cell walls, the exhaustgas introduction cells includes first exhaust gas introduction cells andsecond exhaust gas introduction cells, each of the second exhaust gasintroduction cells has the cross sectional area larger than the crosssectional area of each of the first exhaust gas introduction cells inthe cross section perpendicular to the longitudinal direction of theplurality of cells, and each of the exhaust gas emission cells has thecross sectional area equal to or larger than the cross sectional area ofeach of the second exhaust gas introduction cells in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,and wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas introduction cellsand the exhaust gas emission cells have one of a first structure suchthat the exhaust gas introduction cells and the exhaust gas emissioncells each have the polygonal shape, a side forming the cross sectionalshape of each of the first exhaust gas introduction cells faces one ofthe exhaust gas emission cells, a side forming the cross sectional shapeof each of the second exhaust gas introduction cells faces one of theexhaust gas emission cells, and the side of each of the first exhaustgas introduction cells is longer than the side of each of the secondexhaust gas introduction cells, or the exhaust gas introduction cellsand the exhaust gas emission cells each have the polygonal shape, a sideforming the cross sectional shape of each of the first exhaust gasintroduction cells faces one of the exhaust gas emission cells, and noneof sides forming the cross sectional shape of each of the second exhaustgas introduction cells faces the exhaust gas emission cells, and asecond structure such that the exhaust gas introduction cells and theexhaust gas emission cells are each in a shape formed by a curved line,the porous cell walls include first porous cell walls separating thefirst exhaust gas introduction cells and the exhaust gas emission cells,and second porous cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells, and a thicknessof each of the first porous cell walls is smaller than a thickness ofeach of the second porous cell walls.
 22. A honeycomb filter comprising:a plurality of cells through which exhaust gas is to flow and whichinclude exhaust gas introduction cells and exhaust gas emission cells,the exhaust gas introduction cells each having an open end at an exhaustgas introduction side and a plugged end at an exhaust gas emission side,the exhaust gas emission cells each having an open end at the exhaustgas emission side and a plugged end at the exhaust gas introductionside; porous cell walls defining rims of the plurality of cells; anoxidation catalyst supported inside the porous cell walls in an amountof 5 to 60 g/L; the exhaust gas introduction cells and the exhaust gasemission cells each having a uniform cross sectional shape except for aplugged portion in a cross section perpendicular to a longitudinaldirection of the plurality of cells thoroughly from the exhaust gasintroduction side to the exhaust gas emission side; the exhaust gasemission cells having an average cross sectional area larger than anaverage cross sectional area of the exhaust gas introduction cells inthe cross section perpendicular to the longitudinal direction; and atotal volume of the exhaust gas introduction cells being larger than atotal volume of the exhaust gas emission cells, wherein the exhaust gasintroduction cells include first exhaust gas introduction cells andsecond exhaust gas introduction cells, wherein each of the secondexhaust gas introduction cells has a cross sectional area larger than across sectional area of each of the first exhaust gas introduction cellsin the cross section perpendicular to the longitudinal direction of theplurality of cells, wherein each of the exhaust gas emission cells has across sectional area equal to or larger than the cross sectional area ofeach of the second exhaust gas introduction cells in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas introduction cellsand the exhaust gas emission cells are each in a shape formed by acurved line, wherein the porous cell walls includes first porous cellwalls separating the first exhaust gas introduction cells and theexhaust gas emission cells, and second porous cell walls separating thesecond exhaust gas introduction cells and the exhaust gas emissioncells, and wherein a thickness of each of the first porous cell walls issmaller than a thickness of each of the second porous cell walls. 23.The honeycomb filter according to claim 22, wherein the thickness ofeach of the first porous cell walls is 40 to 75% the thickness of eachof the second porous cell walls.
 24. The honeycomb filter according toclaim 22, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the exhaust gasemission cells, the first exhaust gas introduction cells, and the secondexhaust gas introduction cells are round.
 25. The honeycomb filteraccording to claim 22, wherein, in the cross section perpendicular tothe longitudinal direction of the plurality of cells, the exhaust gasemission cells and the second exhaust gas introduction cells each have aconvex square cross section formed by four outwardly curved lines, andwherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the first exhaust gas introductioncells each have a concave square cross section formed by four inwardlycurved lines.
 26. The honeycomb filter according to claim 22, wherein,in the cross section perpendicular to the longitudinal direction of theplurality of cells, the cross sectional area of each of the secondexhaust gas introduction cells is equal in size to the cross sectionalarea of each of the exhaust gas emission cells, and wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, the cross sectional area of each of the firstexhaust gas introduction cells is 20 to 50% size of the cross sectionalarea of each of the second exhaust gas introduction cells.
 27. Thehoneycomb filter according to claim 22, wherein the honeycomb filtercomprises a plurality of honeycomb fired bodies, wherein each of theplurality of honeycomb fired bodies has the exhaust gas emission cells,the first exhaust gas introduction cells, and the second exhaust gasintroduction cells, wherein each of the plurality of honeycomb firedbodies has an outer wall on an outer periphery of each of the pluralityof honeycomb fired bodies, and wherein the plurality of honeycomb firedbodies are combined with one another by adhesive layers residing betweenthe plurality of honeycomb fired bodies.
 28. The honeycomb filteraccording to claim 27, wherein the outer wall has corner portions, andwherein a side, which contacts the outer wall, of each of the exhaustgas introduction cells and the exhaust gas emission cells adjacent tothe outer wall is straight and parallel to a side corresponding to anouter periphery of the outer wall in a manner that a thickness of theouter wall is uniform except for the corner portions in the crosssection perpendicular to the longitudinal direction of the plurality ofcells.
 29. The honeycomb filter according to claim 22, wherein thehoneycomb filter comprises honeycomb fired bodies, and wherein thehoneycomb fired bodies include one of silicon carbide andsilicon-containing silicon carbide.
 30. The honeycomb filter accordingto claim 22, wherein the porous cell walls have a thickness of 0.10 to0.46 mm.
 31. The honeycomb filter according to claim 22, wherein theporous cell walls have pores having an average pore diameter of 8 to 25μm.
 32. The honeycomb filter according to claim 22, wherein the porouscell walls have a porosity of 40 to 70%.
 33. The honeycomb filteraccording to claim 22, further comprising: a periphery coat layerprovided on a periphery of the honeycomb filter.